Orientation sensing apparatus and a method for determining an orientation

ABSTRACT

An orientation sensing apparatus includes a rotatable first member, an orientation sensor device fixed with the first member for generating orientation data relating to an orientation sensor device orientation, and a rotation sensor device for generating rotation data relating to rotation of the first member. A method for determining an orientation includes assembling a set of sensor data including the orientation data and rotation data and processing the set of sensor data to generate a set of corrected orientation data. The set of corrected orientation data includes the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the first member and misalignment of the gravity sensor device. One or more sets of corrected orientation data may be processed to determine an inclination orientation of the first member or a toolface orientation of a second member which is connected with the first member.

TECHNICAL FIELD

An orientation sensing apparatus and a method for determining an orientation.

BACKGROUND OF THE INVENTION

Many apparatus and methods exist for sensing and determining an orientation of an object in three dimensional space. Such apparatus and methods may be used for surveying, mapping, navigation and other purposes.

The word “orientation” refers to the position of an object with respect to some reference point or coordinate system. For example, azimuthal orientation typically refers to horizontal deviation of an object with respect to a standard reference direction such as north or south. Inclination orientation typically refers to vertical deviation of an object with respect to a standard reference direction such as gravity.

Orientation may also be expressed with respect to a direction or coordinate system which is referenced to the object itself. For example, in drilling and surveying of oil and gas wells, it is usual to refer to the longitudinal axis of a pipe string as the “z-axis” and to refer to the plane which is transverse to the pipe string and the z-axis as the “x-y plane”, regardless of the azimuthal orientation or the inclination orientation of the pipe string and its components.

In this coordinate system, the orientation of the z-axis of the pipe string relative to north is often referred to as the “azimuth” of the pipe string and is typically expressed in degrees from true north or magnetic north, the orientation of the z-axis of the pipe string relative to gravity is often referred to as the “inclination” of the pipe string and is typically expressed in degrees from vertical (gravity), and the orientation of the pipe string relative to the x-y plane is often referred to as the “toolface” of the pipe string or of one of its components and is typically expressed in degrees from the “high side” or “low side” of the pipe string.

More particularly, in the case of “toolface” the orientation is typically expressed in degrees from high side or low side of the pipe string with respect to a reference position or “home position” associated with the pipe string.

Measuring instruments for use in determining orientation must provide an appropriate reference against which the orientation is measured. For example, azimuthal orientation may be measured using magnetic reference instruments such as magnetometers, which use the earth's magnetic field as the reference, or inertial reference instruments such as gyroscopes, which use a predetermined base orientation as the reference. Inclination orientation may be measured using gravity reference instruments such as accelerometers, which use the earth's gravitational field as the reference. Toolface orientation which is expressed relative to high side or low side may also be measured using gravity reference instruments such as accelerometers. Each type of measuring instrument has inherent limitations.

For example, magnetic reference instruments are generally unaffected by external forces exerted on the instruments due to movement such as vibration, but magnetic reference instruments are typically unsuitable for use in proximity with iron, steel and other magnetic materials, since local magnetic fields in such materials will distort magnetic measurements. Magnetic reference instruments are also ineffective for determining inclination orientation.

Inertial reference instruments can provide a relatively accurate measurement of both azimuthal orientation and inclination orientation and are not affected by local magnetic fields, but inertial reference instruments tend to be relatively fragile and can be affected by external forces exerted on the instruments, which can distort inertial measurements. Inertial reference instruments are therefore typically unsuitable for use in harsh environments which include excessive heat, pressure and vibration.

Gravity reference instruments are relatively robust and reliable and can be used in proximity with magnetic materials such as steel pipe. However, gravity reference instruments are affected by external forces exerted on the instruments, which forces can distort the measurements obtained by gravity reference instruments. As a result, the use of gravity reference instruments is often avoided in environments where significant and/or random movement of the instruments can be expected.

Magnetic reference instruments, inertial reference instruments and gravity reference instruments are all used for a wide range of applications in many different industries, including the oil and gas industry.

Due to their characteristics and limitations, inertial reference instruments are often used in wellbore surveying and logging applications but are seldom used in drilling applications.

Magnetic reference instruments and gravity reference instruments are both used extensively in drilling applications as components of a measurement-while-drilling survey system. In such a system, magnetic reference instruments are used to measure azimuthal orientation of the drill string, while gravity reference instruments are used to measure inclination orientation and toolface orientation of the drill string. The magnetic reference instruments are typically isolated from local magnetic fields by being mounted in non-magnetic lengths of drilling pipe.

Ideally, both the magnetic reference instruments and the gravity reference instruments are located in a non-rotating section of the drill string so that the measurements are not affected by rotation of the drill string. In sliding drilling applications where the drill bit is rotated by a power source such as a motor and the drill string is non-rotating, the instruments may be located in any section of the non-rotating drill string. In rotary drilling applications where the drill bit is rotated at least in part by rotation of the drill string, rotation of the drill string may be stopped temporarily in order to facilitate orientation measurements.

Alternatively, the instruments may be located in a non-rotating housing mounted on the drill string so that the instruments do not rotate while the drill string rotates. Such a non-rotating housing is often an integral component of a rotary steerable drilling assembly, so that the housing can function as both a steering mechanism for the drill bit and a mounting location for the orientation instruments. Although this configuration is attractive for facilitating orientation measurements, it is in practice relatively difficult to transmit data pertaining to the orientation measurements up the drill string to the surface, since the data must cross over from the non-rotating housing to the rotating drill string in order to be transmitted up the drill string by a measurement-while-drilling telemetry system.

As a result, it would be preferable in applications involving a rotating drill string to avoid locating the orientation instruments on a separate non-rotating housing. Since magnetic reference instruments are not directly affected by movement of the drill string, the incorporation of magnetic reference instruments in a rotating drill string is relatively straightforward. However, since gravity reference instruments are affected by any movement of the drill string and by the resulting forces that may be exerted on the gravity reference instruments, the incorporation of gravity reference instruments in a rotating drill string is relatively complicated, particularly where the gravity reference instruments are to be relied upon for determining inclination orientation and toolface orientation of the drill string.

Attempts have been made to incorporate gravity reference instruments into rotating pipe strings or drill strings.

U.S. Pat. No. 4,958,125 (Jardine et al) describes a method and apparatus for determining the instantaneous rotation speed of a drill string in a borehole by measuring the centripetal acceleration of the drill string at least two opposite ends of a drill string diameter. The centripetal acceleration is measured in opposite directions at each end of the drill string diameter so that the effects of lateral shock or lateral vibration of the drill string can be eliminated from the centripetal acceleration measurements and the value of the centripetal acceleration is used to calculate the rotation speed of the drill string. In an alternate embodiment, the centripetal acceleration of the drill string at the four opposite ends of two perpendicular drill string diameters in the same cross-section of the drill string is measured so as to obtain four values for the centripetal acceleration, from which the centripetal acceleration can be eliminated and the amplitude and direction of the lateral shock can be derived. Jardine et al does not describe or suggest using accelerometers to determine inclination orientation or toolface orientation of a rotating member.

U.S. Pat. No. 6,065,219 (Murphey et al) describes a method and apparatus for estimating the cross-sectional shape and orientation of a borehole and the motion of a tool therein. In its simplest form, the apparatus in Murphey et al is comprised of a rotatable tool which has a plurality of distance sensors such as acoustic calipers, mechanical calipers or electrical resistance sensors for generating standoff signals representative of standoff distances from each of the distance sensors to respective points on the borehole at a plurality of measurement times and at least one angle sensor such as a magnetometer, inclinometer, accelerometer or gyroscope for generating sinusoidal rotational orientation signals representative of the rotational orientation angle of the tool with respect to a reference direction at the plurality of measurement times. Preferably the tool is comprised of at least one gravity-type sensor and at least one magnetic-type sensor so that a satisfactory angle signal is acquired for any orientation of the axis of the tool. The apparatus is further comprised of a signal processor for calculating an estimate of the actual cross-sectional shape and orientation of the borehole based upon the standoff signals and the rotational orientation signals.

In the preferred embodiment, the rotatable tool in Murphey et al is further comprised of a plurality of accelerometers (preferably four) for generating raw acceleration signals proportional to the lateral translational acceleration of the tool, similar to the invention described in Jardine et al. The raw acceleration signals are filtered to eliminate the contribution of gravity to the raw acceleration signals, thus producing filtered acceleration signals which are processed to provide derived displacement signals which are representative of the lateral translational displacement of the tool in the borehole, which derived displacement signals are compared with measured displacement signals obtained from the standoff signals in order to generate error signals which are representative of a primary error function relating to the difference between the estimated shape of the borehole and the actual shape of the borehole.

Murphey et al does not describe or suggest correcting the orientation signals from the angle sensor or sensors to account for the effects of movement of the tool. Murphey et al also does not describe or suggest obtaining an indication of toolface orientation of the tool described in Murphey et al.

There remains a need for an orientation sensing apparatus and for a method for determining an orientation which uses one or more gravity reference instruments and which can be used in an environment where the gravity reference instruments may be subjected to rotation.

SUMMARY OF THE INVENTION

The present invention relates to an orientation sensing apparatus and to a method for determining an orientation, wherein the orientation sensing apparatus is comprised of an orientation sensor device which in turn comprises one or more gravity reference instruments. The method and the apparatus can be used in an environment where the orientation sensor device may be subjected to rotation, since the apparatus and the method provide for reducing the effects of rotation from the data obtained from the orientation sensor device. The apparatus and method also provide for reducing the effects of misalignment of the orientation sensor device, which misalignment can exacerbate the effects of rotation.

The orientation sensor device may be comprised of any type of gravity reference instrument which is capable of generating the orientation data. Preferably the type of gravity reference instrument is an accelerometer, where “accelerometer” is defined as any device which is capable of measuring acceleration and/or forces due to acceleration, including gravity and/or forces due to gravity.

The orientation sensor device is preferably configured to generate the orientation data in three directions or dimensions so that an orientation can be determined regardless of the relative position of the first member and the orientation sensor device. As a result, the orientation sensor device is preferably comprised of at least three accelerometers.

The orientation sensing apparatus and method may be used to determine an inclination orientation, a toolface orientation or preferably both an inclination orientation and a toolface orientation.

In an apparatus aspect the invention is an orientation sensing apparatus, the apparatus comprising:

-   -   (a) a rotatable first member having a first member orientation         with respect to gravity;     -   (b) an orientation sensor device fixed with the first member for         generating orientation data relating to an orientation sensor         device orientation of the orientation sensor device with respect         to gravity, wherein the orientation sensor device orientation is         referenced to the first member orientation, wherein the         orientation sensor device is comprised of a first accelerometer         for sensing a first sensed component in a first direction,         wherein the orientation sensor device is further comprised of a         second accelerometer for sensing a second sensed component in a         second direction, wherein the orientation sensor device is         further comprised of a third accelerometer for sensing a third         sensed component in a third direction, and wherein the first         direction, the second direction and the third direction are         substantially perpendicular to each other; and     -   (c) a rotation sensor device for generating rotation data         relating to rotation of the first member.

The rotatable first member may be comprised of any shape. Preferably the first member is comprised of a generally circular or cylindrical structure such as a pipe, rod or shaft. More preferably the first member is comprised of a component of a pipe string such as a length of threaded pipe or a length of continuous tubing, or the first member is comprised of a component of a tool which may be connected with or into a pipe string. In preferred embodiments, the first member is comprised of a tubular member such as a shaft or mandrel which is a component of a bottom hole assembly in a pipe string used for drilling. In a particular preferred embodiment the first member is comprised of a shaft or other rotating component of a rotary steerable tool for connection in a pipe string used for drilling.

The first member has an axis of rotation. Preferably the first member defines an x-y plane perpendicular to the axis of rotation and a z-axis coincident with the axis of rotation of the first member.

The orientation sensor device may be fixed with the first member in any suitable manner. Preferably the orientation sensor device is removably mounted in or on the first member so that the orientation sensor device can be removed for servicing, repair or replacement. The orientation sensor device may be mounted in the first member so that the accelerometers are located along the z-axis of the first member or so that the accelerometers are offset from the z-axis. In addition, the orientation sensor device may be oriented at any orientation relative to the x-y plane and the z-axis.

Preferably, however, the orientation sensor device is mounted in or on the first member so that the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane, so that the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane, and so that the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis. This configuration enables the first accelerometer and the second accelerometer to sense inclination substantially relative to the x-y plane and enables the third accelerometer to sense inclination substantially relative to the z-axis.

The rotation sensor device may be comprised of any apparatus or device which is capable of generating rotation data relating to rotation of the first member. The rotation data is used to correct the orientation data to reduce the effects of rotation of the first member and misalignment of the orientation sensor device and may provide an indication of speed of rotation, tangential acceleration due to stick-slip and other causes, centrifugal acceleration, or an indication of some other parameter which assists in defining the rotational movement of the first member. The rotation data may be comprised of one indication or may be comprised of more than one indication.

Preferably the rotation sensor device is capable of generating rotation data which can be related to the orientation data. In other words, preferably the rotation sensor device is capable of generating rotation data which is representative of the rotation of the first member at the time that the orientation data is generated. The rotation sensor device may generate rotation data which is representative of an average value of a parameter relating to rotation of the first member. More preferably the rotation sensor device is capable of generating the rotation data on a continuous real-time basis or on demand so that the rotation data is representative of an instantaneous value of a parameter relating to rotation of the first member.

As one example, the rotation sensor device may be comprised of a counter which monitors the movement of a magnet or some other marker relative to a fixed position, in which case the rotation sensor device provides an average value of a parameter relating to rotation of the first member. As a second example, the rotation sensor device may be comprised of a magnet which rotates with the first member and which induces a varying electrical current in a conductor such as a coil which is positioned adjacent to the first member.

Preferably, however, the rotation sensor device is comprised of one or more accelerometers which are configured to generate rotation data which relates to forces exerted on the rotation sensor device due to rotation of the first member.

The rotation sensor device may provide an indication of tangential acceleration, in which case the rotation sensor device may be comprised of two accelerometers which are oriented to provide sensed components in substantially opposite directions which are both substantially tangential to the rotation of the first member. The accelerometers comprising the rotation sensor device may be independent of the orientation sensor device. Preferably, however, the orientation sensor device and the rotation sensor device share at least one accelerometer where the rotation sensor device provides an indication of tangential acceleration.

As a result, where the rotation sensor device provides an indication of tangential acceleration, preferably the rotation sensor device is comprised of the first accelerometer and a fourth accelerometer for sensing a fourth gravity component in a fourth direction. The fourth accelerometer may be located along the z-axis but is preferably offset from the z-axis. Preferably the first accelerometer and the fourth accelerometer are offset from the z-axis by substantially equal distances. The fourth accelerometer is preferably oriented so that the fourth direction is substantially opposite to the first direction in order that the indication of tangential acceleration may be determined by adding the outputs from the first accelerometer and the fourth accelerometer and dividing by two.

The rotation sensor device may provide an indication of centrifugal acceleration, in which case the rotation sensor device may be comprised of two accelerometers which are oriented to provide sensed components in substantially opposite directions which are both substantially within the x-y plane and which substantially intersect the z-axis. The orientation sensor device and the rotation sensor device may share at least one accelerometer if the first accelerometer and/or the second accelerometer are not offset from the z-axis. Preferably, however, the rotation sensor device is independent of the orientation sensor device where the rotation sensor device provides an indication of centrifugal acceleration.

As a result, where the rotation sensor device provides an indication of centrifugal acceleration, preferably the rotation sensor device is comprised of a fifth accelerometer for sensing a fifth gravity component in a fifth direction and a sixth accelerometer for sensing a sixth gravity component in a sixth direction. The fifth accelerometer and the sixth accelerometer may be located along the z-axis but are preferably offset from the z-axis. Preferably the fifth accelerometer and the sixth accelerometer are offset from the z-axis by substantially equal distances. The fifth accelerometer and the sixth accelerometer are preferably oriented so that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis and such that the sixth direction is substantially opposite to the fifth direction so that the indication of centrifugal acceleration may be determined by adding the outputs from the fifth accelerometer and the sixth accelerometer and dividing by two.

Where the orientation sensor device and the rotation sensor device are both comprised of accelerometers, the orientation sensor device and the rotation sensor device are preferably comprised of three pairs of accelerometers, wherein the three pairs of accelerometers are comprised of the first accelerometer, the second accelerometer, the third accelerometer, the fourth accelerometer, the fifth accelerometer and the sixth accelerometer. Each pair of accelerometers is preferably comprised of two accelerometers which are oriented to provide sensed components which are substantially perpendicular to each other.

The three pairs of accelerometers may be spaced around a circumference of the first member so that each of the three pairs of accelerometers is offset from the z-axis. Preferably the pairs of accelerometers are spaced apart from each other substantially by multiples of ninety degrees so that the preferred relative orientations of the accelerometers as described above can be achieved.

The apparatus of the invention as described above may be used to determine an inclination orientation of the first member as the first member orientation. Preferably, however, the apparatus is also configured to determine a toolface orientation. The toolface orientation may be an orientation of a location or position on the first member relative to gravity.

Preferably, however, the apparatus further comprises a second member, wherein the first member is connected with the second member such that the first member is rotatable relative to the first member, and the toolface orientation is a second member orientation of a location or position on the second member relative to gravity and is preferably relative to the x-y plane. As a result, preferably the apparatus further comprises a referencing mechanism for referencing the second member orientation to the first member orientation so that the sets of corrected orientation data can be used to determine the second member orientation.

The second member may be comprised of any shape and may be connected with the first member in any manner which facilitates rotation of the first member relative to the second member. Preferably the second member is comprised of a sleeve or housing which surrounds the first member so that the first member extends through the second member. In preferred embodiments the second member is comprised of a sleeve or housing which surrounds a portion of a pipe string of the type used for drilling or is comprised of a component of a tool which may be connected with or in the pipe string. In particular preferred embodiments the second member is comprised of a housing for a rotary steerable tool, which housing may be comprised of a device for inhibiting rotation of the second member relative to a borehole.

The second member orientation of the second member may be relative to a home position of the second member. The home position of the second member may be arbitrary or may relate to a specific structure or feature associated with the apparatus or the second member. The second member orientation of the second member may also be relative to a “high side” or a “low side” of the second member relative to gravity.

The referencing mechanism may be comprised of any structure, device or apparatus which is capable of referencing the second member orientation to the first member orientation as the first member rotates relative to the second member.

For example, the referencing mechanism may be comprised of a “high side” or a “low side” indicator associated with the first member and/or the second member.

Preferably the referencing mechanism is comprised of a home position indicator fixed with the second member, wherein the home position indicator is referenced to a home position of the second member. Preferably the referencing mechanism is further comprised of a home position sensor fixed with the first member such that the home position sensor senses the home position indicator as the first member rotates relative to the second member. Preferably the home position sensor is referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the home position indicator and the home position sensor.

The apparatus is configured to assemble sets of sensor data, wherein each set of sensor data is comprised of orientation data generated by the orientation sensor device and rotation data generated by the rotation sensor device. Preferably the orientation data and the rotation data in a set of sensor data are generated substantially contemporaneously so that the rotation data is substantially representative of the rotation of the first member at the time when the orientation data is generated.

The apparatus may be configured so that the apparatus assembles a set of sensor data each time the home position sensor senses the home position indicator, thus generating one set of sensor data for each revolution of the first member.

Preferably, however, the referencing mechanism is comprised of a plurality of incremental position indicators fixed with the second member, wherein each of the incremental position indicators is associated with an incremental position of the second member and wherein each of the incremental positions is referenced to a home position of the second member. Preferably the referencing mechanism is further comprised of an incremental position sensor fixed with the first member such that the incremental position sensor senses the incremental position indicators as the first member rotates relative to the second member, wherein the incremental position sensor is referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the incremental position indicators and the incremental position sensor.

The apparatus may be configured so that the apparatus assembles a set of sensor data each time the incremental position sensor senses one of the incremental position indicators, thus generating a plurality of sets of sensor data for each revolution of the first member. The number of sets of sensor data which are generated for each revolution of the first member may be further enhanced if the incremental position sensor comprises more than one incremental position sensor assembly.

For example, the incremental position sensor may be comprised of a first incremental sensor assembly and a second incremental sensor assembly which are spaced from each other such that the incremental sensor assemblies sense the incremental position indicators sequentially as the first member rotates relative to the second member, in which case the apparatus may be configured so that the apparatus generates a set of sensor data each time one of the incremental sensor assemblies senses one of the incremental position indicators, thus doubling the number of sets of sensor data which are generated for each revolution of the first member.

The incremental position sensor may also be configured to be capable of sensing the direction of the relative rotation between the first member and the second member. For example, the first incremental sensor assembly or the second incremental sensor assembly may be comprised of two sensors which are spaced from each other such that the two sensors sense the incremental position indicators sequentially as the first member rotates relative to the second member. By sensing which of the two sensors senses the incremental position indicators first, the direction of relative rotation can be determined.

The home position indicator, the home position sensor, the incremental position indicators and the incremental position sensor may be comprised of any suitable indicator and sensor combination. Preferably the indicators are comprised of magnets and preferably the sensors are comprised of magnetic sensors. The magnetic sensors may be comprised of any suitable type of magnetic sensor, but preferably are comprised of Hall effect sensors.

The sets of sensor data may be managed in any suitable manner. For example, the sets of sensor data may be processed in order to generate the sets of corrected orientation data. The sets of corrected orientation data may be further processed to determine the first member orientation and/or the second member orientation. The sets of sensor data, the sets of corrected orientation data, the first member orientation and/or the second member orientation may be stored in a memory. The processing may be performed by one or more processors and the memory may be comprised of one or more memory devices.

The apparatus may be connected with a communication system for transmitting the sets of sensor data to a location remote of the apparatus so that none of the processing and memory storage is performed by the apparatus. Preferably, however, at least a portion of the processing is performed by the apparatus and preferably the apparatus is provided with some memory storage capability.

The apparatus is therefore preferably comprised of at least one processor and at least one memory device. The processor and/or the memory may be associated with the first member, the second member or may be located remote of the first member and the second member, but still a part of the apparatus. Preferably the memory and at least one processor are both associated with the first member and are preferably fixed with the first member.

Preferably the memory is configured so that the sets of sensor data may be stored in the memory either before or after an amount of processing so that the sets of sensor data may be linked to the referencing mechanism and thus to the second member orientation. Preferably the memory is comprised of a number of bins which corresponds to the number of sets of orientation data generated during one revolution of the first member. More specifically, preferably the sets of sensor data are linked to one of the incremental sensor assemblies and to one of the incremental position indicators by being stored in bins which correspond to the incremental sensor assemblies and the incremental position indicators.

The sets of sensor data may be processed in order to generate sets of corrected orientation data from the orientation data and the rotation data, wherein the sets of corrected orientation data are comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the first member and the effects of misalignment of the accelerometers.

The sets of corrected orientation data may be further processed to determine the first member orientation and/or to determine the second member orientation by referencing the second member orientation to the first member orientation with the referencing mechanism. The sets of corrected orientation data may be processed individually or one or more sets of corrected orientation data may be processed together.

Preferably the sets of corrected orientation data each comprise a corrected first sensed component corresponding to the first accelerometer, a corrected second sensed component corresponding to the second accelerometer, and a corrected third sensed component corresponding to the third accelerometer. The first member orientation may be determined from the corrected orientation data using basic trigonometry and may be expressed as an inclination orientation relative to vertical or as a toolface orientation of the first member relative to a reference associated with the first member. The second member orientation may also be determined from the corrected orientation data using basic trigonometry and may be expressed as a toolface orientation of the home position relative to high side, low side or some other reference.

The memory may store the sets of sensor data, the sets of corrected orientation data, the first member orientation, the second member orientation, or combinations thereof. If the memory stores the sets of sensor data, the sets of sensor data may be delivered from the memory to a processor for processing to produce the sets of corrected orientation data. If the memory stores the sets of corrected orientation data, the sets of sensor data may be delivered first to a processor for processing to produce the sets of corrected orientation data which in turn may deliver the sets of corrected orientation data to the memory.

Preferably the apparatus is comprised of a first processor, wherein the first processor is configured to process the sets of sensor data in order to generate the sets of corrected orientation data. Preferably the memory stores the sets of corrected orientation data. The sets of corrected orientation data may then be retrieved from the memory for processing in order to determine the first member orientation and/or the second member orientation.

The sets of corrected orientation data may be communicated to a processor remote from the apparatus in order to determine the first member orientation and/or the second member orientation. Alternatively, the apparatus may be comprised of a second processor which is configured to process the sets of corrected orientation data in order to determine the first member orientation and/or the second member orientation. The second processor may be located on the first member and/or the second member, or may be remote from the first member and the second member.

In a preferred embodiment the apparatus is associated with a tool which is connected with or in a pipe string of the type used for drilling. More specifically, in a preferred embodiment the apparatus is associated with a rotary steerable tool as a component of a drill string, wherein the rotary steerable tool facilitates steering of a drill bit while the drill string rotates.

In this preferred embodiment the second member is comprised of a housing, the first member is comprised of a shaft extending through the housing, the first member orientation is comprised of a shaft inclination orientation, and the second member orientation is comprised of a housing toolface orientation. The shaft is comprised of a lower end which is adapted to be connected with a drill bit and an upper end which is adapted to be connected with a drill string. The housing is comprised of a device for inhibiting rotation of the housing relative to a borehole. A steering mechanism is associated with the housing and the shaft, which steering mechanism is adapted to direct the drill bit in a steering direction.

The method of the invention may be performed using the apparatus of the invention or may be performed using a different apparatus. Preferably the method is performed using the apparatus of the invention.

In a method aspect the invention is a method for determining an orientation using an apparatus comprising a rotatable first member having a first member orientation with respect to gravity, wherein the apparatus is further comprised of a orientation sensor device fixed with the first member for generating orientation data relating to a orientation sensor device orientation of the orientation sensor device with respect to gravity, wherein the orientation sensor device orientation is referenced to the first member orientation, and wherein the apparatus is further comprised of a rotation sensor device for generating rotation data relating to rotation of the first member, the method comprising:

-   -   (a) assembling a set of sensor data comprising the orientation         data and the rotation data; and     -   (b) processing the set of sensor data to generate a set of         corrected orientation data, wherein the set of corrected         orientation data is comprised of the orientation data which has         been corrected using the rotation data to reduce the effects of         rotation of the first member and misalignment of the orientation         sensor device.

The sensor data assembling step may be comprised of any step, method or technique for assembling the orientation data and the rotation data. Preferably the orientation data and the rotation data comprising a set of sensor data are generated substantially contemporaneously.

The orientation data is preferably generated by a plurality of accelerometers which are configured to generate the orientation data in a plurality of directions or dimensions so that an orientation can be determined regardless of the relative position of the first member and the orientation sensor device. More preferably the orientation data is generated by at least three accelerometers.

Preferably the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, a second accelerometer for sensing a second sensed component in a second direction, and a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.

The first member has an axis of rotation. The first member defines an x-y plane perpendicular to the axis of rotation and a z-axis coincident with the axis of rotation. Preferably the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member. Preferably the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member. Preferably the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis.

The sensor data processing step is performed in order to reduce the effects of rotation of the first member and misalignment of the orientation sensor device in the orientation data and thus generate the set of corrected orientation data.

The effects of rotation of the first member may be present in the set of orientation data due to several causes. First, to the extent that the speed of rotation of the first member is increasing or decreasing, tangential acceleration of the first member may affect the orientation data from the first accelerometer and the second accelerometer. Second, to the extent that the third accelerometer is not exactly aligned axially with the z-axis of the first member, tangential acceleration of the first member as described above may also affect the orientation data from the third accelerometer. Third, to the extent that the first accelerometer, the second accelerometer and the third accelerometer are not exactly radially aligned relative to the z-axis of the first member, centrifugal acceleration of the first member may affect the orientation data from the first accelerometer, the second accelerometer and the third accelerometer.

The effects of misalignment of the accelerometers may be present in the sets of sensor data due to radial misalignment and/or axial misalignment of the accelerometers. If the third accelerometer is not exactly aligned axially with the z-axis of the first member, this axial misalignment will result in an inaccurate third sensed component. If the first accelerometer, the second accelerometer and the fourth accelerometer are not exactly axially aligned with the x-y plane, this axial misalignment will result in an inaccurate first sensed component, second sensed component and fourth sensed component respectively. If the first accelerometer, the second accelerometer, the third accelerometer and the fourth accelerometer are not exactly aligned radially with respect to the first member, this radial misalignment will result in an inaccurate first sensed component, second sensed component, third sensed component and fourth sensed component respectively which will include a component due to centrifugal acceleration.

The sensor data processing step may therefore be comprised of processing the set of sensor data in order to reduce one or more of these effects.

In preferred embodiments of the method, the rotation data may therefore be comprised of an indication of tangential acceleration of the first member and/or an indication of centrifugal acceleration of the first member.

The indication of tangential acceleration of the first member and/or the indication of centrifugal acceleration of the first member may be obtained in any manner to generate the rotation data.

Preferably the indication of tangential acceleration is obtained from two sensed components from two accelerometers which are both offset from the z-axis and are oriented such that the direction of the two sensed components is substantially within the x-y plane, substantially tangential to the rotation of the first member and substantially opposite to each other. In a preferred embodiment the indication of tangential acceleration is obtained from the first accelerometer and from a fourth accelerometer as described with respect to the apparatus of the invention.

Preferably the indication of centrifugal acceleration is obtained from two sensed components from two accelerometers which are both offset from the z-axis and are oriented such that the direction of the two sensed components is substantially within the x-y plane, substantially intersecting the z-axis and substantially opposite to each other. In a preferred embodiment the indication of centrifugal acceleration is obtained from a fifth accelerometer and a sixth accelerometer as described with respect to the apparatus of the invention.

The indication of tangential acceleration may be used to correct the orientation data in order to account for the effects of tangential acceleration on the orientation data from the first accelerometer and the second accelerometer. The indication of tangential acceleration may also be used to correct the orientation data in order to account for an axial misalignment of the third accelerometer relative to the z-axis. The indication of centrifugal acceleration may be used to correct the orientation data in order to account for a radial misalignment of the first accelerometer, the second accelerometer and the third accelerometer relative to the z-axis. The indication of centrifugal acceleration may also be used to correct the fourth sensed component from the fourth accelerometer in order to account for a radial misalignment of the fourth accelerometer relative to the z-axis. The orientation data may be corrected to account for axial misalignment of the first accelerometer, the second accelerometer and the fourth accelerometer by using comparisons of the first sensed component, the second sensed component and the fourth sensed component with the third sensed component.

The method may be further comprised of the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a first radial misalignment factor for the first accelerometer, establishing a second radial misalignment factor for the second accelerometer, and establishing a third radial misalignment factor for the third accelerometer, wherein the radial misalignment factors describe relationships between the indication of centrifugal acceleration and the sensed components from the first accelerometer, the second accelerometer and the third accelerometer. The calibrating step may also be comprised of establishing a fourth radial misalignment factor for the fourth accelerometer. Preferably the radial misalignment factors are each expressed as a ratio of a change in the sensed component to a change in the indication of centrifugal acceleration.

The radial misalignment factors may be determined in any suitable manner which will relate the sensed components to the indication of centrifugal acceleration. Preferably the radial misalignment factors are determined by subjecting the first member to rotation at varying speeds of rotation while maintaining a constant inclination orientation of the first member, measuring the sensed components from the accelerometers, and determining the relationship between the sensed components and the indication of centrifugal acceleration.

The calibration step may alternatively or also be comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor describes a relationship between the third sensed component and the first sensed component from the first accelerometer, a second axial misalignment factor wherein the second axial misalignment factor describes a relationship between the third sensed component and the second sensed component from the second accelerometer, and optionally a fourth axial misalignment factor wherein the fourth axial misalignment factor describes a relationship between the third sensed component and the fourth sensed component from the fourth accelerometer. Preferably the first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor are each expressed as a ratio of a change in the first sensed component, the second sensed component and the fourth sensed component respectively to a change in the third sensed component.

The first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor may be determined in any suitable manner which will relate the sensed components from the first accelerometer, the second accelerometer and the fourth accelerometer to the third sensed component. The first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor may be determined by subjecting the first member to very slow rotation at varying inclination orientations of the first member, measuring the sensed components from the first accelerometer, the second accelerometer, the third accelerometer and the fourth accelerometer throughout an entire revolution of the first member at each of the inclination orientations, determining the average values of the first sensed component, the second sensed component, the third sensed component and the fourth sensed component for each inclination orientation, and determining the relationship between the average values of each of the first sensed component, the second sensed component and the fourth sensed component and the average values of the third sensed component.

The calibration step may alternatively or also be comprised of establishing a third axial misalignment factor for the third accelerometer, wherein the third axial misalignment factor describes a relationship between the indication of tangential acceleration and the third sensed component from the third accelerometer. Preferably the third axial misalignment factor is expressed as a ratio of a change in the third sensed component to a change in the indication of tangential acceleration or as a ratio of the maximum value of the third sensed component during rotation of the first member.

The third axial misalignment factor may be determined in any suitable manner which will relate the third sensed component to the indication of tangential acceleration. The third axial misalignment factor may be determined by subjecting the first member to rotation while maintaining a constant inclination orientation of the first member, measuring the third sensed component throughout an entire revolution of the first member, and determining the magnitude of the amplitude of the third sensed component as it varies sinusoidally during rotation of the first member. Alternatively, the third axial misalignment factor may be determined by subjecting the first member to rotation under conditions in which the first member will experience tangential acceleration, measuring the sensed components from the third accelerometer and from the first accelerometer and the fourth accelerometer, and determining the relationship between the third sensed component and the indication of tangential acceleration determined from the first sensed component and the fourth sensed component.

The sensor data processing step may be performed iteratively in order to generate the set of corrected orientation data so that both the radial misalignment factors and the axial misalignment factors may be taken into account.

The method may be further comprised of the step of processing one or more sets of corrected orientation data to determine the first member orientation. The step of determining the first member orientation may be comprised of determining the inclination orientation of the x-y plane relative to gravity by using one or more sets of corrected orientation data and principles of trigonometry. The step of determining the first member orientation may be further comprised of determining the toolface orientation of the first member relative to a reference associated with the first member.

The apparatus may be further comprised of a second member having a second member orientation relative to gravity, wherein the first member is connected with the second member such that the first member is rotatable relative to the second member, and the method may be further comprised of the step of processing one or more sets of corrected orientation data to determine the second member orientation.

The step of determining the second member orientation may be comprised of referencing the second member orientation to the first member orientation so that one or more sets of corrected orientation data may be used to determine the second member orientation. The second member orientation may be determined from the first member orientation or may be determined directly from one or more sets of corrected orientation data. Preferably the second member orientation is determined as the toolface orientation of the second member in the x-y plane by using one or more sets of corrected orientation data and principles of trigonometry.

The apparatus may be further comprised of a referencing mechanism for referencing the second member orientation to the first member orientation.

The sensor data assembling step may be comprised of any step, method or technique for assembling the orientation data and the rotation data. Preferably the orientation data and the rotation data comprising a set of sensor data are generated substantially contemporaneously.

The sensor data assembling step may be comprised of assembling a set of sensor data each time a home position sensor associated with the referencing mechanism senses a home position indicator associated with the referencing mechanism. Alternatively or additionally, the sensor data assembling step may be comprise of assembling a set of sensor data each time an incremental position sensor associated with the referencing mechanism senses one of a plurality of incremental position indicators associated with the referencing mechanism. The incremental position sensor may be comprised of a plurality of incremental sensor assemblies and the sensor data assembling step may be comprised of assembling a set of sensor data each time one of the incremental sensor assemblies.

In a preferred embodiment the method of the invention is performed using an apparatus which is associated with a tool which is connected with or in a pipe string of the type used for drilling. More specifically, in a preferred embodiment the apparatus is associated with a rotary steerable tool as a component of a drill string, wherein the rotary steerable tool facilitates steering of a drill bit while the drill string rotates.

In this preferred embodiment the first member orientation is comprised of an inclination orientation of the x-y plane relative to gravity and the second member orientation is comprised of a toolface orientation of the second member in the x-y plane.

In this preferred embodiment the second member is comprised of a housing, the first member is comprised of a shaft extending through the housing, the first member orientation is comprised of a shaft inclination orientation, and the second member orientation is comprised of a housing toolface orientation. The shaft is comprised of a lower end which is adapted to be connected with a drill bit and an upper end which is adapted to be connected with a drill string. The housing is comprised of a device for inhibiting rotation of the housing relative to a borehole. A steering mechanism is associated with the housing and the shaft, which steering mechanism is adapted to direct the drill bit in a steering direction.

In this preferred embodiment either or both of the shaft inclination orientation and the housing toolface orientation may be determined from the set of corrected orientation data, and the housing orientation determining step may be comprised of referencing the second member orientation to the first member orientation. The first member orientation may be further comprised of a shaft toolface orientation so that the housing orientation determining step may be comprised of referencing the housing toolface orientation to the shaft toolface orientation.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side view of a rotary steerable tool attached to a lower end of a drill string, including an orientation sensing apparatus according to a preferred embodiment of the invention.

FIG. 2 is a longitudinal cross-section view of the portion of the rotary steerable tool of FIG. 1 which includes the orientation sensing apparatus.

FIG. 3 is a transverse cross-section view of the orientation sensing apparatus of FIG. 1, taken along line III-III of FIG. 2, depicting the orientation sensor device and the rotation sensor device.

FIG. 4 is a schematic pictorial view of the orientation sensor device and the rotation sensor device of FIG. 3, depicting the directions of the sensed components of each of the accelerometers.

FIG. 5 is an isolated pictorial cutaway view of the referencing mechanism of the orientation sensing apparatus of FIG. 1, depicting the primary referencing mechanism and the secondary referencing mechanism.

FIG. 6 is an isolated side view of the referencing mechanism of the orientation sensing apparatus of FIG. 1, depicting the primary referencing mechanism and the secondary referencing mechanism.

FIG. 7 is a transverse cross-section view taken along line VII-VII of FIG. 6, depicting the primary referencing mechanism.

FIG. 8 is a transverse cross-section view taken along line VIII-VIII of FIG. 6, depicting the secondary referencing mechanism.

DETAILED DESCRIPTION

The present invention is an orientation sensing apparatus and a method for determining an orientation.

In a preferred embodiment, the apparatus is included as a component in a drill string and is configured to determine an inclination orientation and a toolface orientation. More specifically, in the preferred embodiment the apparatus is included as a component of a rotary steerable tool.

Referring to FIG. 1, a rotary steerable tool (20) is connected to a lower end of a drill string (22). The drill string (22) may be comprised of lengths of jointed drill pipe and/or drill collars connected together or may be comprised of a continuous length of pipe such as a coiled tubing.

The rotary steerable tool (20) may be comprised of a point the bit tool or a push the bit tool and may include any mechanism for pointing or pushing a drill bit (24). A preferred rotary steerable tool (20) for use with the invention is described in U.S. Pat. No. 6,769,499 (Cargill et al).

The rotary steerable tool (20) includes a first member comprising a shaft (26) and a second member comprising a housing (28). The shaft (26) is connected with the housing (28) so that the shaft (26) is rotatable relative to the housing (28).

The shaft (26) has a lower end (30) which is connected with the drill bit (24) and an upper end (32) which is connected with the drill string (22).

The housing (28) is comprised of an anti-rotation device (34) for inhibiting rotation of the housing (28) relative to a borehole (not shown) when the rotary steerable tool (20) is in use.

The rotary steerable tool (20) is further comprised of a steering mechanism (36) which is associated with the shaft (26) and the housing (28) and which is capable of directing the drill bit (24) in a steering direction by deflecting the shaft (26) either to point the drill bit (24) or to push the drill bit (24). The steering mechanism (36) may be comprised of a fixed deflection of the shaft (26) but preferably the steering mechanism (36) is comprised of an actuatable mechanism for deflecting the shaft (26) to provide the steering direction.

The drill string (22) is comprised of a drill string communication system (38) for providing uplink and/or downlink communication between the drill string (22) and the surface (not shown). The drill string communication system is preferably comprised of a measurement-while-drilling (MWD) communication system such as an mud pulse, acoustic or electromagnetic MWD system.

An orientation sensing apparatus (40) according to the invention is included as a component of the rotary steerable tool (20). In its most basic form, the apparatus (40) is comprised of the shaft (26), a orientation sensor device (42) and a rotation sensor device (44) and is capable of providing a determination of inclination orientation of the shaft (26) relative to gravity.

In the preferred embodiment, the apparatus (40) is further comprised of the housing (28) and a referencing mechanism (46) and is capable of providing both the determination of inclination orientation of the shaft (26) and a determination of toolface orientation of the housing (28) relative to gravity.

Referring to FIGS. 2-4, the shaft (26) has an axis of rotation. The shaft (26) defines an x-y plane (48) which is perpendicular to the axis of rotation and a z-axis (50) which is coincident with the axis of rotation.

Referring to FIGS. 2-4, the orientation sensor device (42) is comprised of a first accelerometer (52) for sensing a first sensed component in a first direction (54), a second accelerometer (56) for sensing a second sensed component in a second direction (58), and a third accelerometer (60) for sensing a third sensed component in a third direction (62).

In the preferred embodiment the accelerometers (52,56,60) are oriented so that the first direction (54), the second direction (58) and the third direction (62) are substantially perpendicular to each other. The first accelerometer (52) is offset from the z-axis (50) and is oriented such that the first direction (54) is substantially within the x-y plane (48) and is substantially tangential to the rotation of the shaft (26). The second accelerometer (56) is offset from the z-axis (50) and is oriented such that the second direction (58) is substantially within the x-y plane (48) and is substantially tangential to the rotation of the shaft (26). The third accelerometer (60) is offset from the z-axis (50) and is oriented such that the third direction (62) is substantially parallel with the z-axis (50).

In the preferred embodiment the rotation sensor device (44) is configured to provide an indication of tangential acceleration of the shaft (26) and an indication of centrifugal acceleration of the shaft (26).

In order to provide the indication of tangential acceleration, the rotation sensor device (44) in the preferred embodiment is comprised of the first accelerometer (52) and a fourth accelerometer (64) for sensing a fourth sensed component in a fourth direction (66). The fourth accelerometer (64) is offset from the z-axis (50) and is oriented such that the fourth direction (66) is substantially opposite to the first direction (54). The indication of tangential acceleration can be determined by adding the first sensed component and the fourth sensed component together and then dividing the sum by two.

In order to provide the indication of centrifugal acceleration, the rotation sensor device (44) in the preferred embodiment is further comprised of a fifth accelerometer (68) for sensing a fifth sensed component in a fifth direction (70) and a sixth accelerometer (72) for sensing a sixth sensed component in a sixth direction (74). The fifth accelerometer (68) and the sixth accelerometer (72) are oriented such that the fifth direction (70) and the sixth direction (74) are substantially within the x-y plane (48) and substantially intersect the z-axis (50) and such that the sixth direction (74) is substantially opposite to the fifth direction (70). The indication of centrifugal acceleration can be determined by adding the fifth sensed component and the sixth sensed component together and then dividing the sum by two.

As indicated, in the preferred embodiment each of the accelerometers (52,56,60,64,68,72) is offset from the z-axis (50). This is due to the impracticality of locating the accelerometers (52,56,60,64,68,72) on the z-axis (50). If, however, the accelerometers (52,56,60,64,68,72) are located on the z-axis (50) and are perfectly aligned, the need to correct the orientation data to reduce the effects of rotation of the shaft (26) and misalignment of the accelerometers (52,56,60,64,68,72) is eliminated.

As depicted in FIG. 3 the accelerometers (52,56,60,64,68,72) are mounted on or in cavities located on the outer circumference of the shaft (26) and are removable so that the orientation sensor device (42) and the rotation sensor device (44) comprising the accelerometers (52,56,60,64,68,72) can be removed for servicing, repair and/or replacement. In the preferred embodiment each of the accelerometers (52,56,60,64,68,72) is offset from the z-axis (50) by substantially the same distance.

Exemplary but non-limiting suitable parameters for the accelerometers (52,56,60,64,68,72) include a measurement range of between about ±5 G and about ±25 G, a clock input frequency of about 1.0 MHz and a sample rate frequency of about 1000 Hz, which provide a theoretical resolution of about 0.01 G for a ±5 G accelerometer and about 0.05 G for a ±25 G accelerometer. It has been found that suitable accelerometers for use in the invention are manufactured by Silicon Designs, Inc of Issaquah, Wash., U.S.A.

In the preferred embodiment the accelerometers (52,56,60,64,68,72) are configured as a first pair (75) of accelerometers, a second pair (77) of accelerometers and a third pair (79) of accelerometers. The three pairs (75,77,79) of accelerometers are preferably each configured as three separate two-axis accelerometer modules so that the directions of the sensed components from each pair of accelerometers are substantially perpendicular. The three pairs (75,77,79) of accelerometers are also spaced apart around the circumference of the shaft (26) substantially by multiples of ninety degrees.

Referring to FIGS. 3-4, in the preferred embodiment the first pair (75) of accelerometers is comprised of the first accelerometer (52) and the fifth accelerometer (68), the second pair (77) of accelerometers is comprised of the second accelerometer (56) and the third accelerometer (60), and the third pair (79) of accelerometers is comprised of the fourth accelerometer (64) and the sixth accelerometer (72). The first pair (75) of accelerometers is spaced by substantially ninety degrees from the second pair (77) of accelerometers and the first pair (75) of accelerometers is spaced by substantially one hundred eighty degrees from the third pair (79) of accelerometers.

As a result of this configuration of the accelerometers (52,56,60,64,68,72), the first direction (54), the second direction (58) and the third direction (62) are substantially perpendicular to each other, the first direction (54) and the fourth direction (66) are substantially opposite to each other, and the fifth direction (70) and the sixth direction (74) are substantially opposite to each other. In addition, the first direction (54), the second direction (58) and the fourth direction (66) are substantially within the x-y plane (48) and are substantially tangential to the rotation of the shaft (26), the third direction (62) is substantially parallel to the z-axis (50), and the fifth direction (70) and the sixth direction (74) are substantially within the x-y plane (48), substantially intersect the z-axis (50), and are substantially opposite to each other.

The orientation sensor device (42) generates orientation data relating to a orientation sensor device orientation of the orientation sensor device (42) relative to gravity. The rotation sensor device (44) generates rotation data relating to rotation of the shaft (26). The orientation data and the rotation data are generated as sets of sensor data, wherein each set of sensor data is comprised of orientation data and rotation data. In the preferred embodiment the orientation data and the rotation data are generated substantially contemporaneously so that each set of sensor data is comprised of orientation data and rotation data generated at a particular common point in time.

The sets of sensor data may be processed to generate sets of corrected orientation data, wherein the sets of corrected orientation data are comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the shaft (26) and the effects of misalignment of the accelerometers (52,56,60,64).

The sets of corrected orientation data may be further processed to determine a first member orientation, which may be expressed as the shaft inclination orientation, which is the inclination orientation of the shaft (26). The first member orientation may also be expressed as the shaft toolface orientation, which is the instantaneous toolface orientation of the shaft (26). Using the referencing mechanism (46), the sets of corrected orientation data may also be further processed to determine a second member orientation, which may be expressed as a housing toolface orientation, which is the toolface orientation of the housing (28).

The sets of corrected orientation data may be processed individually to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28). In the preferred embodiment, sets of corrected orientation data are combined and processed together to determine the inclination orientation of the shaft (26) the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28).

The sets of sensor data may be generated randomly or according to a schedule. In the preferred embodiment, the sets of sensor data are generated in association with the referencing mechanism (46) such that the referencing mechanism (46) controls the generation of the sets of sensor data, which facilitates the determination of the shaft toolface orientation and the housing toolface orientation.

Referring to FIG. 2 and FIGS. 5-8, the referencing mechanism (46) references a position of the shaft (26) to a position of the housing (28) so that the position of the housing (28) relative to gravity can be determined from the position of the shaft (26). More particularly, the referencing mechanism (46) provides the position of a fixed location on the shaft (26) relative to a fixed location on the housing (28) so that the toolface orientation of the shaft (26) can be referenced to the toolface orientation of the housing (28).

In the preferred embodiment the referencing mechanism (46) is comprised of a home position indicator (76) which is fixed with the housing (28). In the preferred embodiment the home position indicator (76) is comprised of a magnet. The home position indicator (76) is referenced to a home position (78) of the housing (28) so that the position of the home position indicator (76) relative to the home position (78) is either known or can be determined. The home position (78) of the housing (28) may be arbitrary or may relate to a specific structure or feature associated with the housing (28).

The referencing mechanism (46) is further comprised of a home position sensor (80) which is fixed with the shaft (26). In the preferred embodiment the home position sensor (80) is comprised of a Hall effect sensor, but other types of magnetic sensors may also be used. As the shaft (26) rotates relative to the housing (28), the home position sensor (80) senses the home position indicator (76). The home position sensor (80) is referenced to the orientation sensor device orientation of the orientation sensor device (42).

As a result, the second member orientation is referenced to the first member orientation by the home position indicator (76) and the home position sensor (80). More specifically, the toolface orientation of the housing (28) is referenced to the toolface orientation of the shaft (26) by the home position indicator (76) and the home position sensor (80).

In the preferred embodiment the referencing mechanism (46) is further comprised of a plurality of incremental position indicators (82) which are fixed with the housing (28). Each of the incremental position indicators (82) is preferably comprised of a magnet. Each of the incremental position indicators (82) is associated with an incremental position of the housing (28) and each of the incremental positions is referenced to the home position (78) of the housing (28) so that the position of the incremental position indicators (82) relative to the home position (78) is either known or can be determined. In the preferred embodiment the apparatus (40) includes eight incremental position indicators (82) spaced evenly around the inner circumference of the housing (28), thus providing eight incremental positions of the housing (28).

In the preferred embodiment the referencing mechanism (46) is also further comprised of an incremental position sensor (84) fixed with the shaft (26). The incremental position sensor (84) is preferably comprised of a first incremental sensor assembly (86) and a second incremental sensor assembly (88), which are spaced from each other and sense the incremental position indicators (82) sequentially as the shaft (26) rotates relative to the housing (28). The incremental sensor assemblies (86,88) are both preferably comprised of Hall effect sensors, but other types of magnetic sensors may be used.

In the preferred embodiment the use of two incremental sensor assemblies (86,88) results in the incremental position sensor (84) sensing the incremental position indicators (82) sixteen times during one revolution of the shaft (26) relative to the housing (28), since each of the incremental sensor assemblies (86,88) will sense each of the eight incremental position indicators (82) once during one revolution of the shaft (26) relative to the housing (28). This effectively provides sixteen relative positions between the shaft (26) and the housing (28) during one revolution of the shaft (26) relative to the housing (28).

In the preferred embodiment the first incremental sensor assembly (86) is preferably further comprised of two Hall effect sensors which are spaced from each other such that the two sensors sense the incremental position indicators (82) sequentially as the shaft (26) rotates relative to the housing (28). The use of two magnetic sensors in the first incremental sensor assembly (86) provides additional functionality to the referencing mechanism (46), since the direction of the rotation of the shaft (26) relative to the housing (28) can be determined by considering which of the two magnetic sensors in the first incremental sensor assembly (86) first senses a particular incremental position indicator (82).

The incremental position sensor (84) comprising the incremental sensor assemblies (86,88) is referenced to the orientation sensor device orientation of the orientation sensor device (42).

As a result, the second member orientation is referenced to the first member orientation by the incremental position indicators (82) and the incremental sensor assemblies (86,88). More particularly, the toolface orientation of the housing (28) is referenced to the toolface orientation of the shaft (26) by the incremental position indicators (82) and the incremental sensor assemblies (86,88).

Referring to FIGS. 5-6, the home position indicator (76) and the home position sensor (80) are located at a first axial position on the apparatus (40) and provide a primary referencing mechanism (90). The incremental position indicators (82) and the incremental sensor assemblies (86,88) are located at a second axial position on the apparatus (40) and provide a secondary referencing mechanism (92). The second axial position is spaced from the first axial position so that the secondary referencing mechanism (92) does not interfere with the primary referencing mechanism (90).

Either or both of the primary referencing mechanism (90) and the secondary referencing mechanism (92) may be used to reference the toolface orientation of the housing (28) to the toolface orientation of the shaft (26).

The apparatus (40) may be configured so that the apparatus (40) assembles a set of sensor data each time the home position sensor (80) senses the home position indicator (76), with the result that one set of sensor data is assembled during each revolution of the shaft (26) relative to the housing (28). This in turn facilitates one determination of the inclination orientation of the shaft (26) and one determination of the toolface orientation of the housing (28) for each revolution of the shaft (26) relative to the housing (28).

In the preferred embodiment the apparatus (40) is configured so that the apparatus (40) assembles a set of sensor data each time one of the incremental sensor assemblies (86,88) senses one of the incremental position indicators (82), with the result that sixteen sets of sensor data are assembled during each revolution of the shaft (26) relative to the housing (28). This in turn facilitates sixteen determinations of the inclination orientation of the shaft (26) and sixteen determinations of the toolface orientation of the housing (28) for each revolution of the shaft (26) relative to the housing (28).

The sets of sensor data are processed to generate sets of corrected orientation data. The sets of corrected orientation data are used directly or indirectly to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and the toolface orientation of the housing (28).

Referring to FIG. 2, in the preferred embodiment the apparatus (40) is further comprised of a first processor (94) which is configured to process the sets of sensor data in order to generate the sets of corrected orientation data. The first processor (94) is mounted on or in the shaft (26) adjacent to the orientation sensor device (42) and the rotation sensor device (44) so that the first processor (94) rotates with the shaft (26).

The apparatus (40) may optionally be further comprised of a second processor (96) which is configured to process the sets of corrected orientation data in order to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28). The second processor (96) may be mounted on or in the shaft (26) or may be located remote of the apparatus (40). As depicted in FIG. 2, the second processor (96) is located adjacent to the first processor (94). Alternatively, the second processor (96) may be located at the surface of the borehole and may either be a component of the apparatus (40) or may be separate from the apparatus (40).

Referring to FIG. 2, in the preferred embodiment the apparatus (40) is also further comprised of a memory (98) for storing the sets of corrected orientation data after they have been generated by the first processor (94). Where applicable, the memory (98) may also store the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28) as determined by the second processor (96). The memory (98) is mounted on or in the shaft (26) adjacent to the orientation sensor device (42) and the rotation sensor device (44) so that the memory (98) rotates with the shaft (26).

In the preferred embodiment the sets of corrected orientation data are stored in the memory (98) so that each of the sets of corrected orientation data may be linked to one of the first incremental sensor assembly (86) and the second incremental sensor assembly (88) and to one of the incremental position indicators (82).

More particularly, in the preferred embodiment the memory (98) is comprised of a number of bins (not shown) which corresponds to the number of sets of corrected orientation data generated during one revolution of the shaft (26), so that the sets of corrected orientation data are linked to one of the first incremental sensor assembly (86) and the second incremental sensor assembly (88) and to one of the incremental position indicators (82) by being stored in bins which correspond to the incremental sensor assemblies (86,88) and to the incremental position indicators (82). The sets of corrected orientation data corresponding to a particular incremental sensor assembly (86,88) and a particular incremental position indicator (82) are combined together in the bins of the memory (98) so that the sets of corrected orientation data can be averaged before processing to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) or the toolface orientation of the housing (28).

Referring to FIG. 2, the shaft (26) may be comprised of an electronics sub or an electronics insert so that the components of the apparatus (40) may be mounted together on or within the electronics sub or insert.

In the preferred embodiment the memory (98) is electrically connected with the drill string communication system (38) so that the sets of corrected orientation data may be communicated to the surface. Alternatively, the first processor (94) and/or the second processor (96) may be electrically connected directly with the drill string communication system (38).

The apparatus (40) therefore facilitates the use of the orientation sensor device (42) in a rotating environment while enabling the determination of inclination orientation and toolface orientation with respect to gravity. The apparatus (40) avoids the transfer of data between a rotating component and a non-rotating component because the orientation data is generated on the shaft (26) and not on the housing (28) and is subsequently processed by the first processor (94), stored in the memory (98) and communicated to the drill string communication system (38) along the shaft (26) and the drill string (22) without crossing over to a non-rotating component.

The preferred embodiment of the method of the invention is performed using the preferred embodiment of the apparatus (40) of the invention.

In the preferred embodiment of the method of the invention a set of sensor data is assembled and is then processed to generate a set of corrected orientation data, wherein the set of corrected orientation data is comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the shaft (26) and the effects of misalignment of the accelerometers (52,56,60,64).

More particularly, in the preferred embodiment of the method of the invention the orientation data is corrected to reduce the effects of tangential acceleration and centrifugal acceleration upon the orientation data. These effects may be due to stick-slip of the shaft (26) which results in tangential acceleration of the shaft (26), may be due to misalignment of the accelerometers (52,56,60,64,68,72) or may be due to a combination of tangential acceleration and misalignment of the accelerometers (52,56,60,64,68,72).

Tangential acceleration of the shaft (26) will result in inaccurate sensed components from the first accelerometer (52), the second accelerometer (56) and the fourth accelerometer (64) since these accelerometers (52,56,64) are oriented so that the sensed components are substantially tangential to the rotation of the shaft (26).

Mechanical misalignment of the accelerometers (52,56,60,64,68,72) results from the accelerometers (52,56,60,64,68,72) not being oriented exactly in their intended directions, but instead being oriented only “substantially” in their intended directions. As one example, the first direction (54), the second direction (58) and the third direction (62) may not be exactly perpendicular to each other. As a second example, the first direction (54), the second direction (58) and the fourth direction (66) may not be exactly within the x-y plane (48) and/or may not be exactly tangential to the rotation of the shaft (26). As a third example, the third direction (62) may not be exactly parallel to the z-axis (50). As a fourth example, the fifth direction (70) and the sixth direction (74) may not be exactly within the x-y plane (48) and/or may not exactly intersect the z-axis (50).

In addition to mechanical misalignment of the accelerometers (52,56,60,64,68,72) as described above, cross axial sensitivity of the accelerometers (52,56,60,64,68,72) also potentially affects the intended sensing directions of the accelerometers (52,56,60,64,68,72) so that cross axial sensitivity of the accelerometers (52,56,60,64,68,72) may be considered as an effect of misalignment of the accelerometers (52,56,60,64,68,72). Cross axial sensitivity is the sensitivity of a accelerometer (52,56,60,64,68,72) to input in a direction which is transverse to its intended measuring direction.

A radial sensed component of the accelerometers (52,56,60,64,68,72) may be defined as a portion of a sensed component which is in a radial direction relative to the shaft (26). A radial sensed component may be due to centrifugal acceleration resulting from rotation of the shaft (26). A radial sensed component may also be due to lateral movement or lateral vibration of the shaft (26).

Radial misalignment of one of the accelerometers (52,56,60,64,68,72) exists where a accelerometer is oriented such that its sensed component includes an erroneous radial sensed component. The erroneous radial sensed component may be an unwanted radial sensed component where the accelerometer is not intended to sense a radial sensed component or may be a reduced radial sensed component where the accelerometer is intended to sense only a radial sensed component.

An erroneous radial sensed component may be very significant in the sensed components of the accelerometers (52,56,60,64,68,72), since rotation and lateral movement of the shaft (26) may produce centrifugal acceleration and/or lateral acceleration values which significantly exceed 1 G, while the gravity portion of the sensed components is always less than or equal to 1 G. In addition, a radial sensed component due to rotation of the shaft (26) will increase as the rotation speed of the shaft (26) increases.

Radial misalignment of the first accelerometer (52), the second accelerometer (56), the third accelerometer (60) and the fourth accelerometer (64) will produce an unwanted radial sensed component, since these accelerometers (52,56,60,64) are oriented so that their sensed components are substantially perpendicular to the direction of the centrifugal acceleration of the shaft (26) and any radial component in their sensed components is therefore unwanted. The magnitude of the unwanted radial sensed component may be very significant relative to the gravity portions of the sensed components of these accelerometers (52,56,60,64).

Radial misalignment of the fifth accelerometer (68) and the sixth accelerometer (72) will produce a reduced radial sensed component, since these accelerometers (68,72) are oriented so that their sensed components are substantially only in the radial direction, with the result that any radial misalignment will cause a reduction in their sensed components. Small amounts of radial misalignment of the fifth accelerometer (68) and the sixth accelerometer (72) do not significantly affect the accuracy of the sensed components of these accelerometers (68,72).

An axial sensed component of the accelerometers (52,56,60,64,68,72) may be defined as any portion of a sensed component which is not in a radial direction relative to the shaft (26) and which relates to the axes defined by the x-y plane (48) and the z-axis (50). An axial sensed component is primarily due to the effects of gravity on the accelerometers (52,56,60,64,68,72), but may also be due to tangential acceleration of the shaft (26) due to stick-slip and other causes.

Axial misalignment of one of the accelerometers (52,56,60,64) exists where a accelerometer is oriented such that its sensed component includes axial sensed components along more than one axis, thus resulting in a sensed component which represents the vector sum of axial sensed components along two or more axes.

Axial misalignment is most significant where it results in an erroneous axial sensed component in a direction which is tangential to the rotation of the shaft (26), since tangential acceleration of the shaft (26) can significantly exceed 1 G, while the gravity portion of the sensed components from these accelerometers (52,56,60,64) is always less than 1 G.

Axial misalignment of the third accelerometer (60) will therefore be significant since the third accelerometer (60) is oriented so that its sensed component is substantially parallel to the z-axis (50), with the result that any axial misalignment will result in an erroneous axial sensed component which is tangential to the rotation of the shaft (26).

Axial misalignment of the first accelerometer (52), the second accelerometer (56) and the fourth accelerometer (64) may, however, also significantly affect the accuracy of the sensed components of these accelerometers (52,56,64), since these sensed components are intended primarily to include gravity components, with the result that axial misalignment will cause these gravity components to be relatively inaccurate.

As a result, in the preferred embodiment of the invention the orientation data is corrected to reduce the effects of tangential acceleration in the first sensed component and the second sensed component, to reduce the centrifugal acceleration effects of radial misalignment in the first sensed component, the second sensed component and the third sensed component, to reduce the tangential acceleration effects of axial misalignment in the third sensed component, and to reduce the effects of axial misalignment in the first sensed component and the second sensed component. The rotation data may also be corrected to reduce the effects of radial misalignment and axial misalignment of the fourth accelerometer (64).

In order to correct the orientation data to generate the sets of corrected orientation data in the preferred embodiment, a set of misalignment factors must be available for the orientation sensor device (42) and the rotation sensor device (44). These misalignment factors are unique to a particular apparatus (40) and are preferably obtained by calibration of the apparatus (40).

The set of misalignment factors pertaining to the orientation data from the orientation sensor device (42) includes a first radial misalignment factor for the first accelerometer (52), a second radial misalignment factor for the second accelerometer (56), a third radial misalignment factor for the third accelerometer (60), a first axial misalignment factor for the first accelerometer (52), a second axial misalignment factor for the second accelerometer (56), and a third axial misalignment factor for the third accelerometer (60).

The set of misalignment factors may also include a fourth radial misalignment factor for the fourth accelerometer (64) and a fourth axial misalignment factor for the fourth accelerometer (64) in order to provide for correction of the rotation data from the rotation sensor device (44).

The radial misalignment factors may be obtained by determining the relationship between centrifugal acceleration and the sensed components of the accelerometers (52,56,60,64). More particularly, the radial misalignment factors may be obtained by determining the relationships between indications of centrifugal acceleration obtained from the fifth accelerometer (68) and the sixth accelerometer (72) and the sensed components from each of the accelerometers (52,56,60,64) at varying rotation speeds of the shaft (26).

The unwanted radial sensed component in the sensed components from the accelerometers (52,56,60,64) due to radial misalignment of the accelerometers (52,56,60,64) will vary linearly with variations in the indication of centrifugal acceleration from the accelerometers (68,72). As a result, the radial misalignment factors may be expressed as a ratio of a change in the sensed components from the accelerometers (52,56,60,64) to a change in the indication of centrifugal acceleration from the accelerometers (68,72).

The varying sensed components from the accelerometers (52,56,60,64) and the varying indications of centrifugal acceleration from the accelerometers (68,72) may be obtained by rotating the shaft (26) at different rotation speeds of the shaft (26) while maintaining a constant inclination orientation of the shaft (26).

The sensed components from the accelerometers (52,56,60,64) will vary sinusoidally as the shaft (26) rotates, but will provide average sensed components which are representative of the unwanted radial sensed components. The indications of centrifugal acceleration can be obtained by adding the sensed components from the accelerometers (68,72) and dividing the sum by two. The radial misalignment factors can be obtained graphically as the slope of a plot of the average sensed components from the accelerometers (52,56,60,64) against the indications of centrifugal acceleration from the accelerometers (68,72) at varying rotation speeds of the shaft (26), which represents the ratio of the change in the sensed components from the accelerometers (52,56,60,64) to the change in the indications of centrifugal acceleration from the accelerometers (68,72).

Alternatively, the radial misalignment factors may be obtained by determining the relationships between the average sensed components from the accelerometers (52,56,64) at different rotation speeds of the shaft (26) and known or predicted values for centrifugal acceleration at such rotation speeds, thus eliminating the need for the fifth accelerometer (68) and the sixth accelerometer (72) in the apparatus (40).

The first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor may be obtained by determining the relationships between the third sensed component from the third accelerometer (60) and the sensed components from each of the accelerometers (52,56,64) as the shaft (26) is slowly rotated at varying inclination orientations of the shaft (26).

The unwanted axial sensed components in the sensed components from the accelerometers (52,56,64) due to axial misalignment of the accelerometers (52,56,64) will vary linearly with variations in the third sensed component from the third accelerometer (60). As a result, the first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor may each be expressed as a ratio of a change in the sensed components from the accelerometers (52,56,64) to a change in the third sensed component from the third accelerometer (60).

The varying sensed components from the accelerometers (52,56,60,64) may be obtained by rotating the shaft (26) at very low speed (in order to minimize centrifugal acceleration) while varying the inclination orientation of the shaft (26).

The sensed components from the accelerometers (52,56,60,64) will vary sinusoidally as the shaft (26) rotates, but will provide average sensed components which are representative of the z-axis (50) components of the sensed components. The z-axis (50) components of the sensed components from the accelerometers (52,56,64) represent the unwanted axial sensed components. The first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor can be obtained graphically as the slope of a plot of the average sensed components from the accelerometers (52,56,64) against the average third sensed component from the third accelerometer (60) respectively at different inclination orientations of the shaft (26).

Alternatively, the first axial misalignment factor, the second axial misalignment factor and the fourth axial misalignment factor may be obtained by determining the relationship between the average sensed components from the accelerometers (52,56,64) as the shaft (26) is slowly rotated at different inclination orientations of the shaft (26) and predicted values for the third sensed component at such inclination orientations.

The third axial misalignment factor may be obtained by determining the relationship between tangential acceleration and the third sensed component of the third accelerometer (60). More particularly, the third axial misalignment factor may be obtained by determining the relationship between an indication of tangential acceleration obtained from the first accelerometer (52) and the fourth accelerometer (64) and the third sensed component of the third accelerometer (60) as the tangential acceleration of the shaft (26) is varied.

The unwanted axial sensed component in the third sensed component from the third accelerometer (60) due to axial misalignment of the third accelerometer (60) will vary linearly with variations in the indication of tangential acceleration from the accelerometers (52,64). As a result, the third axial misalignment factor may be expressed as a ratio of a change in the third sensed component from the third accelerometer (60) to a change in the indication of tangential acceleration from the accelerometers (52,64).

The varying third sensed component from the third accelerometer (60) and the varying indications of tangential acceleration from the accelerometers (52,64) may be obtained by rotating the shaft (26) non-uniformly to produce different tangential accelerations while maintaining a constant inclination orientation of the shaft (26).

The third sensed component from the third accelerometer (60) will vary sinusoidally as the shaft (26) rotates, but will provide an average third sensed component which is representative of the unwanted axial sensed component. The indications of tangential acceleration can be obtained by adding the sensed components from the accelerometers (52,64) and dividing the sum by two. The third axial misalignment factor can be obtained graphically as the slope of a plot of the average third sensed component from the third accelerometer (60) against the indications of tangential acceleration from the accelerometers (52,64) at varying tangential accelerations of the shaft (26), which represents the ratio of the change in the third sensed component from the third accelerometer (60) to the change in the indications of tangential acceleration from the accelerometers (52,64).

Alternatively, the third axial misalignment factor may be obtained indirectly by determining the relationship between the third sensed component from the third accelerometer (60) and the expected value of the third sensed component at known orientations of the orientation sensor device (42) relative to gravity.

For example, the third axial misalignment factor may be determined by maintaining the shaft (26) at a constant inclination orientation and then rotating the shaft (26) slowly through a complete revolution so that centrifugal acceleration is minimized. If axial misalignment of the third accelerometer (60) exists, the third sensed component of the third accelerometer (60) will vary sinusoidally as the shaft (26) rotates. The third axial misalignment factor is equal to the amplitude of the third sensed component of the third accelerometer (60) as it varies sinusoidally during rotation of the shaft (26).

Once the set of misalignment factors for the apparatus (40) are known, the method of the invention may be used to generate the orientation data and the rotation data as sets of sensor data and to process the sets of sensor data to generate sets of corrected orientation data. In the preferred embodiment the sets of corrected orientation data are generated from the sets of sensor data by processing the sets of sensor data iteratively.

In the preferred embodiment, the first processor (94) generates the sets of corrected orientation data by processing the sets of sensor data which are generated by the orientation sensor device (42) and the rotation sensor device (44).

In the preferred embodiment, the first step in the method is to generate the orientation data using the orientation sensor device (42) and the rotation data using the rotation sensor device (44) and to assemble the orientation data and the rotation data into sets of sensor data. Each set of sensor data will correspond to one of the first incremental sensor assembly (86) and the second incremental sensor assembly (88) and to one of the incremental position indicators (82).

The sets of sensor data are then processed by the first processor (94) to generate sets of corrected orientation data. The sets of corrected orientation data are comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the shaft (26) and misalignment of the accelerometers (52,56,60,64,68,72).

The effects of rotation of the shaft (26) include centrifugal acceleration caused by rotation of the shaft (26) and tangential acceleration caused by changes in the speed of rotation of the shaft (26). The effects of misalignment of the accelerometers (52,56,60,64,68,72) include both mechanical misalignment of the accelerometers (52,56,60,64,68,72) relative to their intended directions and cross axial sensitivity of the accelerometers (52,56,60,64,68,72) themselves.

The effects of rotation of the shaft (26) and misalignment of the accelerometers (52,56,60,64) together affect the sensed components from the accelerometers (52,56,60,64) in a potentially non-linear manner according to equations having the following general form: X _(M) =G _(X) +F _(X)(C,a,G _(Z))  (1) Y _(M) =G _(Y) +F _(Y)(C,a,G _(Z))  (2) Z _(M) =G _(Z) +F _(Z)(C,a)  (3) X′ _(M) =G _(X′) +F _(X′)(C,a,G _(Z))  (4)

where:

-   -   X_(M), Y_(M), Z_(M), X′_(M) are the first sensed component, the         second sensed component, the third sensed component and the         fourth sensed component respectively;     -   G_(X), G_(Y), G_(Z), G_(X′) are a corrected first sensed         component, a corrected second sensed component, a corrected         third sensed component and a corrected fourth sensed component         respectively;     -   F_(X), F_(Y), F_(Z), F_(X′) are general functions describing the         effects of rotation of the shaft (26) and misalignment of the         gravity sensors (52,56,60,64) upon the sensed components X_(M),         Y_(M), Z_(M), and X′_(M) respectively;     -   C is centrifugal acceleration; and     -   a is tangential acceleration.

The functions F_(X), F_(Y), F_(Z), F_(X′) may be used to develop equations for generating the sets of corrected orientation data from the sets of sensor data. These equations may be linear or non-linear and may incorporate appropriate assumptions and approximations in order to simplify the generation of the sets of corrected orientation data.

Preferably relatively simple linear equations developed from the functions F_(X), F_(Y), F_(Z), F_(X′) are applied to the sets of sensor data in order to generate the sets of corrected orientation data.

In the preferred embodiment, reasonable approximations for G_(X), G_(Y), G_(Z), are obtained by applying linear equations developed from the functions F_(X), F_(Y), F_(Z), F_(X′) according to the following steps of the method after the first step.

The second step in the method is to process each set of sensor data to correct the first sensed component, the second sensed component, the third sensed component and the fourth sensed component for radial misalignment of the accelerometers (52,56,60,64). This step is performed by using the following formulae: X _(M) =X _(RC) +C*R _(X)  (5) Y _(M) =Y _(RC) +C*R _(Y)  (6) Z _(M) =Z _(RC) +C*R _(Z)  (7) X′ _(M) =X′ _(RC) +C*R _(X′)  (8)

where:

-   -   X_(RC), Y_(RC), Z_(RC), X′_(RC) are the first sensed component,         the second sensed component, the third sensed component and the         fourth sensed component respectively, corrected for radial         misalignment; and     -   R_(X), R_(Y), R_(Z), R_(X) are the first radial misalignment         factor, the second radial misalignment factor, the third radial         misalignment factor and the fourth radial misalignment factor         respectively.

The third step in the method is to process each set of sensor data to calculate an initial value of tangential acceleration using the following formulae: X _(RC) =X _(RCZ) +Z _(RC) *A _(X)  (9) X′ _(RC) =X′ _(RCZ) +Z _(RC) *A _(X′)  (10) a=(X _(RCZ) +X′ _(RCZ))/2  (11)

where:

-   -   X_(RCZ), X′_(RCZ) are X_(RC) and X′_(RC) respectively which have         been corrected for axial misalignment using Z_(RC) (which has         not yet been corrected for axial misalignment); and     -   A_(X), A_(X) are the first axial misalignment factor and the         fourth axial misalignment factor respectively.

The fourth step in the method is to process each set of sensor data to correct Z_(RC) for axial misalignment of the third accelerometer (60) using the third axial misalignment factor, using the following formula: Z _(RC) =Z _(RCZ) +a*A _(Z)  (12)

where:

-   -   Z_(RCZ) is Z_(RC) which has been corrected for axial         misalignment of the third accelerometer (60); and     -   A_(Z) is the third axial misalignment factor.

The fifth step in the method is to process each set of sensor data to correct the first sensed component, the second sensed component and the fourth sensed component for axial misalignment of the accelerometers (52,56,64) using the following formulae: X _(RC) =X _(RAC) +Z _(RCZ) *A _(X)  (13) Y _(RC) =Y _(RAC) +Z _(RCZ) *A _(Y)  (14) X′ _(RC) =X′ _(RAC) +Z _(RCZ) *A _(X′)  (15)

where:

-   -   X_(RAC), Y_(RAC), X′_(RAC) are X_(RC), Y_(RC), and X′_(RC) which         have been corrected for axial misalignment using Z_(RCZ) (which         has been corrected for axial misalignment).

The sixth step in the method is to process each set of sensor data to recalculate tangential acceleration using the following formula: a _(COR)=(X _(RAC) +X′ _(RAC))/2  (16)

where:

-   -   a_(COR) is a corrected value of tangential acceleration

The seventh step in the method is to process each set of sensor data to generate a set of corrected orientation data using the following formulae: G _(X) =X _(RAC) −a _(COR)  (17) G _(Y) =Y _(RAC) −a _(COR)  (18) G _(Z) =Z _(RCZ) −a _(COR) *A _(Z)  (19)

Each set of corrected orientation data, comprising the corrected first sensed component, the corrected second sensed component and the corrected third sensed component provides a reasonable approximation of the actual orientation sensor device orientation which is largely independent of tangential acceleration and centrifugal acceleration, but which continues to include a lateral component due to lateral movement or vibration of the shaft (26). This lateral component can be eliminated by combining and averaging of sets of corrected orientation data, as described below.

The sets of corrected orientation data may be processed either individually or in combination to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28). In the preferred embodiment, the sets of corrected orientation data are managed and processed in the following manner in order to determine the inclination orientation of the shaft (26) and/or the toolface orientation of the housing (28).

In the preferred embodiment, sixteen sets of sensor data are assembled during each revolution of the shaft (26) relative to the housing (28), with each set of sensor data comprising orientation data and rotation data generated by the accelerometers (52,56,60,64,68,72).

In the preferred embodiment the sample period for generating the orientation data and the rotation data is about one millisecond. If the first sensed component, the second sensed component or the fourth sensed component approaches the saturation limit of the accelerometers (52,56,60,64,68,72), the set of sensor data is eliminated from consideration.

In the preferred embodiment the sets of corrected orientation data are stored in the memory (98) after being generated by the first processor (94). In the preferred embodiment, the memory (98) includes sixteen bins, with each bin comprising a separate buffer for storing the corrected first sensed component (G_(X)), the corrected second sensed component (G_(Y)) and the corrected third sensed component (G_(Z)).

In the preferred embodiment the memory (98) is configured to store a plurality of sets of corrected orientation data in the three buffers of each of the sixteen bins so that sets of corrected orientation data are combined in the buffers. Average values for the corrected sensed components can be calculated by dividing the sums of the corrected orientation data in the buffers by the number of sets of corrected orientation data stored in the buffers. These average values for the corrected sensed components provide a correction for lateral movement or lateral vibration of the shaft (26).

The average values of the corrected first sensed component (G_(X)), the corrected second sensed component (G_(Y)) and the corrected third sensed component (G_(Z)) contained in each of the sixteen bins are processed to determine inclination orientation of the shaft (26), toolface orientation of the shaft (26), and toolface orientation of the housing (28).

In the preferred embodiment depicted in FIG. 2, the corrected orientation data is communicated from the memory (98) to the second processor (96) so that the corrected orientation data may be processed by the second processor (96) in order to determine the inclination orientation of the shaft (26), the toolface orientation of the shaft (26) and/or the toolface orientation of the housing (28). If the second processor (96) is located at the surface of the borehole instead of adjacent to the first processor (94), the corrected orientation data may be communicated by the drill string communication system (38) from the memory (98) up the borehole to the second processor (96) for processing by the second processor (96). Once the corrected orientation data has been communicated to the second processor (96), the buffers in the memory (98) may be purged so that new corrected orientation data may be stored in the memory (98).

The inclination orientation of the shaft (26) is determined independently for each of the sixteen bins using G_(X), G_(Y), G_(Z), and trigonometry principles. The determinations of inclination orientation for each of the sixteen bins are then averaged together to provide an average inclination orientation of the shaft (26), thus providing a further correction for lateral movement or lateral vibration of the shaft (26).

In order to determine inclination orientation of the shaft (26) for a particular bin, the following formulae are used: G _(TOTAL)=((G _(X) *G _(X))+(G _(Y) *G _(Y))+(G _(Z) *G _(X)))^(0.5)  (20) G _(XY)=((G _(X) *G _(X))+(G _(Y) *G _(Y)))^(0.5)  (21) Inclination=a sin(G _(XY) /G _(TOTAL))  (22)

(if G_(XY)/G_(TOTAL)<0.707 and G_(Z)>0) Inclination=180−a sin(G _(XY) /G _(TOTAL))  (23)

(if G_(XY)/G_(TOTAL)<0.707 and G_(Z)<0) Inclination=a cos(G _(Z) /G _(TOTAL))  (24)

(if G_(XY)/G_(TOTAL)≧0.707)

The toolface orientation of the shaft (26) is determined independently for each of the sixteen bins using G_(X), G_(Y), G_(Z), and trigonometry principles. The toolface orientation of the housing (28) for each of the sixteen bins is determined independently by referencing the toolface orientation of the housing (28) for a particular bin to the toolface orientation of the shaft (26) for the particular bin using the referencing mechanism (46).

More particularly, the toolface orientation of the housing (28) for each of the sixteen bins is determined by adjusting the toolface orientation of the shaft (26) for each bin by the amount of offset between the position of the bin and the home position (78) of the housing (28) as determined by the referencing mechanism (46). The determinations of the toolface orientation of the housing (28) are expressed so that they fall within a range of between 0 degrees and 360 degrees relative to the home position (78) of the housing (28).

The determinations of toolface orientation of the housing (28) for each of the sixteen bins are then averaged together, thus providing a further correction for lateral movement or lateral vibration of the shaft (26).

In order to determine toolface orientation of the shaft (26) for a particular bin, the following formulae are used: Toolface=a sin(G _(X) /G _(XY))  (25)

(if magnitude of G_(X)/G_(XY) is ≦0.707 and G_(Y) is negative) Toolface=a cos(−G _(Y) /G _(XY))  (26)

(if magnitude of G_(Y)/G_(XY) is ≦0.707 and G_(X) is not negative) Toolface=180 degrees−a sin(G _(X) /G _(XY))  (27)

(if magnitude of G_(X)/G_(XY) is ≦0.707 and G_(Y) is not negative) Toolface=−a cos(−G _(Y) /G _(XY))  (28)

(if magnitude of G_(Y)/G_(XY) is ≦0.707 and G_(X) is negative)

The inclination orientation and toolface orientation determinations generated by the second processor (96) may be stored in a further memory (not shown) associated with the second processor (96) or may be transmitted to other locations using a suitable communication device. For example, if the second processor (96) is located at the surface, the inclination orientation and toolface orientation determinations may be transmitted by the drill string communication system (38) back to the apparatus (40) in order to provide feedback to the apparatus (40) in connection with a closed loop steering system.

As described, the apparatus (40) and method of the invention may be used to determine an inclination orientation of a rotating first member relative to gravity and/or to determine a toolface orientation of the first member or of a second member relative to gravity. The inclination orientation and the toolface orientation may be determined for the purpose of monitoring the direction of a borehole while the borehole is being drilled, or the inclination orientation and/or the toolface orientation may be determined for the purpose of steering a drill string (22) during drilling. In the preferred embodiment, the apparatus (40) is comprised of a component of a rotary steerable tool (20) and is used both to monitor the direction of a borehole and either directly as part of a closed loop steering system or indirectly by drilling personnel to assist in steering the drill string (22) during drilling.

In this document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. 

1. An orientation sensing apparatus, the apparatus comprising: (a) a rotatable first member having a first member orientation with respect to gravity; (b) an orientation sensor device fixed with the first member for generating orientation data relating to an orientation sensor device orientation of the orientation sensor device with respect to gravity, wherein the orientation sensor device orientation is referenced to the first member orientation, wherein the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, and wherein the first direction, the second direction and the third direction are substantially perpendicular to each other; (c) a rotation sensor device for generating rotation data relating to rotation of the first member; and (d) a first processor, wherein the apparatus is configured to assemble sets of sensor data, wherein each set of sensor data is comprised of the orientation data and the rotation data, wherein the first processor is configured to process the sets of sensor data in order to generate sets of corrected orientation data each comprising a corrected first sensed component, a corrected second sensed component and a corrected third sensed component, and wherein the sets of corrected orientation data are comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the first member and misalignment of the orientation sensor device.
 2. The apparatus as claimed in claim 1, further comprising a second member having a second member orientation with respect to gravity, wherein the first member is connected with the second member such that the first member is rotatable relative to the second member, and further comprising a referencing mechanism for referencing the second member orientation to the first member orientation.
 3. The apparatus as claimed in claim 2 wherein the referencing mechanism is comprised of a home position indicator fixed with the second member, wherein the home position indicator is referenced to a home position of the second member, wherein the referencing mechanism is further comprised of a home position sensor fixed with the first member such that the home position sensor senses the home position indicator as the first member rotates relative to the second member, and wherein the home position sensor is referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the home position indicator and the home position sensor.
 4. The apparatus as claimed in claim 3 wherein the apparatus is configured so that the apparatus assembles a set of sensor data each time the home position sensor senses the home position indicator.
 5. The apparatus as claimed in claim 3 wherein the home position indicator is comprised of a magnet and wherein the home position sensor is comprised of a magnetic sensor.
 6. The apparatus as claimed in claim 5 wherein the magnetic sensor is comprised of a Hall effect sensor.
 7. The apparatus as claimed in claim 2, further comprising a memory for storing the sets of corrected orientation data.
 8. The apparatus as claimed in claim 7 wherein the first processor and the memory are fixed to the first member.
 9. The apparatus as claimed in claim 8, further comprising a second processor, wherein the second processor is configured to process the sets of corrected orientation data in order to determine at least one of the first member orientation and the second member orientation.
 10. The apparatus as claimed in claim 2 wherein the referencing mechanism is comprised of a plurality of incremental position indicators fixed with the second member, wherein each of the incremental position indicators is associated with an incremental position of the second member, and wherein each of the incremental positions is referenced to a home position of the second member.
 11. The apparatus as claimed in claim 10 wherein the referencing mechanism is further comprised of an incremental position sensor fixed with the first member such that the incremental position sensor senses the incremental position indicators as the first member rotates relative to the second member, and wherein the incremental position sensor is referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the incremental position indicators and the incremental position sensor.
 12. The apparatus as claimed in claim 11 wherein the incremental position indicators are each comprised of a magnet and wherein the incremental position sensor is comprised of a magnetic sensor.
 13. The apparatus as claimed in claim 12 wherein the incremental position sensor is comprised of a first incremental sensor assembly and a second incremental sensor assembly, wherein the first incremental sensor assembly and the second incremental sensor assembly are spaced from each other such that the first incremental sensor assembly and the second incremental sensor assembly sense the incremental position indicators sequentially as the first member rotates relative to the second member.
 14. The apparatus as claimed in claim 13 wherein the first incremental sensor assembly is comprised of two magnetic sensors and wherein the magnetic sensors are spaced from each other such that the magnetic sensors sense the incremental position indicators sequentially as the first member rotates relative to the second member.
 15. The apparatus as claimed in claim 13 wherein the apparatus is configured so that the apparatus assembles a set of sensor data each time the first incremental sensor assembly and the second incremental sensor assembly senses one of the incremental position indicators, wherein each set of sensor data is comprised of the orientation data and the rotation data.
 16. The apparatus as claimed in claim 15 wherein each of the sets of corrected orientation data corresponds to one of the first incremental sensor assembly and the second incremental sensor assembly and to one of the incremental position indicators, further comprising a memory for storing the sets of corrected orientation data, wherein the sets of corrected orientation data are stored in the memory so that each of the sets of corrected orientation data may be linked to the one of the first incremental sensor assembly and the second incremental sensor assembly and to the one of the incremental position indicators.
 17. The apparatus as claimed in claim 16 wherein the first processor and the memory are fixed to the first member.
 18. The apparatus as claimed in claim 17, further comprising a second processor, wherein the second processor is configured to process the sets of corrected orientation data in order to determine at least one of the first member orientation and the second member orientation.
 19. The apparatus as claimed in claim 2 wherein the second member is comprised of a housing, wherein the first member is comprised of a shaft extending through the housing, wherein the first member orientation is comprised of a shaft inclination orientation, and wherein the second member orientation is comprised of a housing toolface orientation.
 20. The apparatus as claimed in claim 19 wherein the shaft is comprised of a lower end and wherein the lower end of the shaft is adapted to be connected with a drill bit.
 21. The apparatus as claimed in claim 20 wherein the shaft is comprised of an upper end and wherein the upper end of the shaft is adapted to be connected with a drilling string.
 22. The apparatus as claimed in claim 21 wherein the housing is comprised of a device for inhibiting rotation of the housing relative to a borehole.
 23. The apparatus as claimed in claim 22, further comprising a steering mechanism associated with the housing and the shaft which is adapted to direct the drill bit in a steering direction.
 24. The apparatus as claimed in claim 23 wherein the referencing mechanism is comprised of a home position indicator fixed with the housing, wherein the home position indicator is referenced to a home position of the housing, wherein the referencing mechanism is further comprised of a home position sensor fixed with the shaft such that the home position sensor senses the home position indicator as the shaft rotates relative to the housing, and wherein the home position sensor is referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the home position indicator and the home position sensor.
 25. The apparatus as claimed in claim 24 wherein the apparatus is configured so that the apparatus assembles a set of sensor data each time the home position sensor senses the home position indicator, wherein each set of sensor data is comprised of the orientation data and the rotation data.
 26. The apparatus as claimed in claim 25, further comprising a memory for storing the sets of corrected orientation data.
 27. The apparatus as claimed in claim 26 wherein the first processor and the memory are fixed to the shaft.
 28. The apparatus as claimed in claim 27, further comprising a second processor, wherein the second processor is configured to process the sets of corrected orientation data in order to determine at least one of the shaft inclination orientation and the housing toolface orientation.
 29. The apparatus as claimed in claim 23 wherein the shaft has an axis of rotation, wherein the shaft defines an x-y plane perpendicular to the axis of rotation, wherein the shaft defines a z-axis coincident with the axis of rotation, wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the shaft, wherein the rotation sensor device is comprised of the first accelerometer and a fourth accelerometer for sensing a fourth sensed component in a fourth direction, and wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction.
 30. The apparatus as claimed in claim 29 wherein the first accelerometer and the fourth accelerometer are offset from the z-axis by substantially equal distances.
 31. The apparatus as claimed in claim 23 wherein the shaft has an axis of rotation, wherein the shaft defines an x-y plane perpendicular to the axis of rotation, wherein the shaft defines a z-axis coincident with the axis of rotation, and wherein the rotation sensor device is comprised of a fifth accelerometer for sensing a fifth sensed component in a fifth direction and a sixth accelerometer for sensing a sixth sensed component in a sixth direction, and wherein the fifth accelerometer and the sixth accelerometer are oriented such that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis, and such that the sixth direction is substantially opposite to the fifth direction.
 32. The apparatus as claimed in claim 31 wherein the fifth accelerometer and the sixth accelerometer are offset from the z-axis.
 33. The apparatus as claimed in claim 31 wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and substantially tangential to the rotation of the shaft, and wherein the rotation sensor device is comprised of the first accelerometer and a fourth accelerometer for sensing a fourth sensed component in a fourth direction, wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction.
 34. The apparatus as claimed in claim 33 wherein the first accelerometer and the fourth accelerometer are offset from the z-axis by substantially equal distances.
 35. The apparatus as claimed in claim 34 wherein the fifth accelerometer and the sixth accelerometer are offset from the z-axis.
 36. The apparatus as claimed in claim 35 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the shaft and wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis.
 37. The apparatus as claimed in claim 36 wherein the orientation sensor device and the rotation sensor device are comprised of three pairs of accelerometers, wherein the three pairs of accelerometers are comprised of the first accelerometer, the second accelerometer, the third accelerometer, the fourth accelerometer, the fifth accelerometer and the sixth accelerometer, wherein the shaft has a circumference, and wherein the three pairs of accelerometers are spaced around the circumference of the shaft.
 38. An orientation sensing apparatus, the apparatus comprising: (a) a rotatable first member having a first member orientation with respect to gravity; (b) an orientation sensor device fixed with the first member for generating orientation data relating to an orientation sensor device orientation of the orientation sensor device with respect to gravity, wherein the orientation sensor device orientation is referenced to the first member orientation, wherein the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, and wherein the first direction, the second direction and the third direction are substantially perpendicular to each other; and (c) a rotation sensor device for generating rotation data relating to rotation of the first member; wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the rotation sensor device is comprised of the first accelerometer and a fourth accelerometer for sensing a fourth sensed component in a fourth direction, and wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction.
 39. The apparatus as claimed in claim 38 wherein the first accelerometer and the fourth accelerometer are offset from the z-axis by substantially equal distances.
 40. An orientation sensing apparatus, the apparatus comprising: (a) a rotatable first member having a first member orientation with respect to gravity; (b) an orientation sensor device fixed with the first member for generating orientation data relating to an orientation sensor device orientation of the orientation sensor device with respect to gravity, wherein the orientation sensor device orientation is referenced to the first member orientation, wherein the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, and wherein the first direction, the second direction and the third direction are substantially perpendicular to each other; and (c) a rotation sensor device for generating rotation data relating to rotation of the first member; wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, and wherein the rotation sensor device is comprised of a fifth accelerometer for sensing a fifth sensed component in a fifth direction and a sixth accelerometer for sensing a sixth sensed component in a sixth direction, and wherein the fifth accelerometer and the sixth accelerometer are oriented such that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis and such that the sixth direction is substantially opposite to the fifth direction.
 41. The apparatus as claimed in claim 40 wherein the fifth accelerometer and the sixth accelerometer are offset from the z-axis.
 42. The apparatus as claimed in claim 40 wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and substantially tangential to the rotation of the first member, and wherein the rotation sensor device is comprised of the first accelerometer and a fourth accelerometer for sensing a fourth sensed component in a fourth direction, wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction.
 43. The apparatus as claimed in claim 42 wherein the first accelerometer and the fourth accelerometer are offset from the z-axis by substantially equal distances.
 44. The apparatus as claimed in claim 43 wherein the fifth accelerometer and the sixth accelerometer are offset from the z-axis.
 45. The apparatus as claimed in claim 44 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member and wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis.
 46. The apparatus as claimed in claim 45 wherein the orientation sensor device and the rotation sensor device are comprised of three pairs of accelerometers, wherein the three pairs of accelerometers are comprised of the first accelerometer, the second accelerometer, the third accelerometer, the fourth accelerometer, the fifth accelerometer and the sixth accelerometer, wherein the first member has a circumference, and wherein the three pairs of accelerometers are spaced around the circumference of the first member.
 47. A method for determining an orientation using an apparatus comprising a rotatable first member having a first member orientation with respect to gravity, wherein the apparatus is further comprised of an orientation sensor device fixed with the first member for generating orientation data relating to an orientation sensor device orientation of the orientation sensor device with respect to gravity, wherein the orientation sensor device orientation is referenced to the first member orientation, and wherein the apparatus is further comprised of a rotation sensor device for generating rotation data relating to rotation of the first member, the method comprising: (a) assembling a set of sensor data comprising the orientation data and the rotation data; and (b) processing the set of sensor data to generate a set of corrected orientation data, wherein the set of corrected orientation data is comprised of the orientation data which has been corrected using the rotation data to reduce the effects of rotation of the first member and misalignment of the orientation sensor device.
 48. The method as claimed in claim 47, further comprising the step of processing corrected orientation data comprising at least one set of corrected orientation data to determine the first member orientation.
 49. The method as claimed in claim 47 wherein the apparatus is further comprised of a second member having a second member orientation with respect to gravity, wherein the first member is connected with the second member such that the first member is rotatable relative to the second member, further comprising the step of comprising the step of processing corrected orientation data comprising at least one set of corrected orientation data to determine the second member orientation.
 50. The method as claimed in claim 49 wherein the step of determining the second member orientation is comprised of referencing the second member orientation to the first member orientation.
 51. The method as claimed in claim 50 wherein the apparatus is further comprised of a referencing mechanism, wherein the referencing mechanism is comprised of a home position indicator referenced to the home position of the second member, wherein the referencing mechanism is further comprised of a home position sensor referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the home position indicator and the home position sensor, and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the home position sensor senses the home position indicator.
 52. The method as claimed in claim 50 wherein the apparatus is further comprised of a referencing mechanism, wherein the referencing mechanism is comprised of a plurality of incremental position indicators referenced to the home position of the second member, wherein the referencing mechanism is further comprised of an incremental position sensor referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the incremental position indicators and the incremental position sensor, and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the incremental position sensor senses one of the incremental position indicators.
 53. The method as claimed in claim 52 wherein the incremental position sensor is comprised of a first incremental sensor assembly and a second incremental sensor assembly and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the first incremental sensor assembly and the second incremental sensor assembly senses one of the incremental position indicators.
 54. The method as claimed in claim 49 wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, and wherein the first member orientation is comprised of an inclination of the x-y plane with respect to gravity.
 55. The method as claimed in claim 54 wherein the second member orientation is comprised of an orientation of the second member in the x-y plane.
 56. The method as claimed in claim 49 wherein the second member is comprised of a housing, wherein the first member is comprised of a shaft extending through the housing, wherein the first member orientation is comprised of a shaft inclination orientation and wherein the second member orientation is comprised of a housing toolface orientation.
 57. The method as claimed in claim 56 wherein the shaft is comprised of a lower end and wherein the lower end of the shaft is adapted to be connected with a drill bit.
 58. The method as claimed in claim 57 wherein the shaft is comprised of an upper end and wherein the upper end of the shaft is adapted to be connected with a drilling string.
 59. The method as claimed in claim 58 wherein the housing is comprised of a device for inhibiting rotation of the housing relative to a borehole.
 60. The method as claimed in claim 59, further comprising a steering mechanism associated with the housing and the shaft which is adapted to direct the drill bit in a steering direction.
 61. The method as claimed in claim 60 wherein the step of determining the second member orientation is comprised of referencing the second member orientation to the first member orientation.
 62. The method as claimed in claim 61 wherein the apparatus is further comprised of a referencing mechanism, wherein the referencing mechanism is comprised of a home position indicator referenced to the home position of the housing, wherein the referencing mechanism is further comprised of a home position sensor referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the home position indicator and the home position sensor, and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the home position sensor senses the home position indicator.
 63. The method as claimed in claim 61 wherein the apparatus is further comprised of a referencing mechanism, wherein the referencing mechanism is comprised of a plurality of incremental position indicators referenced to the home position of the housing, wherein the referencing mechanism is further comprised of an incremental position sensor referenced to the orientation sensor device orientation so that the second member orientation is referenced to the first member orientation by the incremental position indicators and the incremental position sensor, and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the incremental position sensor senses one of the incremental position indicators.
 64. The method as claimed in claim 63 wherein the incremental position sensor is comprised of a first incremental sensor assembly and a second incremental sensor assembly and wherein the sensor data assembling step is comprised of assembling a set of sensor data each time the first incremental sensor assembly and the second incremental sensor assembly senses one of the incremental position indicators.
 65. The method as claimed in claim 60 wherein the rotation data is comprised of an indication of tangential acceleration of the first member.
 66. The method as claimed in claim 65 wherein the shaft has an axis of rotation, wherein the shaft defines an x-y plane perpendicular to the axis of rotation, wherein the shaft defines a z-axis coincident with the axis of rotation, wherein the apparatus is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the shaft, wherein the apparatus is further comprised of a fourth accelerometer for sensing a fourth sensed component in a fourth direction, wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction, and wherein the indication of tangential acceleration is obtained from the first sensed component and the fourth sensed component.
 67. The method as claimed in claim 66 wherein the orientation sensor device is comprised of the first accelerometer, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.
 68. The method as claimed in claim 67 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the first sensed component and the second sensed component of the orientation data using the indication of tangential acceleration in order to account for tangential acceleration of the first member.
 69. The method as claimed in claim 68 wherein the sensor data processing step is further comprised of correcting the first sensed component of the orientation data and correcting the fourth sensed component of the rotation data in order to account for an axial misalignment of the first accelerometer, the second accelerometer and the fourth accelerometer relative to the x-y plane.
 70. The method as claimed in claim 69 wherein the calibrating step is further comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the third sensed component, establishing a second axial misalignment factor for the second accelerometer wherein the second axial misalignment factor is expressed as a ratio of a change in the second sensed component to a change in the third sensed component, and establishing a fourth axial misalignment factor for the fourth accelerometer wherein the fourth axial misalignment factor is expressed as a ratio of a change in the fourth sensed component to a change in the third sensed component.
 71. The method as claimed in claim 67 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the shaft, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the third sensed component of the orientation data using the indication of tangential acceleration in order to account for an axial misalignment of the third accelerometer relative to the z-axis.
 72. The method as claimed in claim 71, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a third axial misalignment factor for the third accelerometer wherein the third axial misalignment factor is expressed as a ratio of a change in the third sensed component to a change in the indication of tangential acceleration.
 73. The method as claimed in claim 60 wherein the rotation data is comprised of an indication of centrifugal acceleration of the first member.
 74. The method as claimed in claim 73 wherein the shaft has an axis of rotation, wherein the shaft defines an x-y plane perpendicular to the axis of rotation, wherein the shaft defines a z-axis coincident with the axis of rotation, wherein the apparatus is further comprised of a fifth accelerometer for sensing a fifth sensed component in a fifth direction, wherein the apparatus is further comprised of a sixth accelerometer for sensing a sixth sensed component in a sixth direction, wherein the fifth accelerometer and the sixth accelerometer are oriented such that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis and such that the sixth direction is substantially opposite to the fifth direction, and wherein the indication of centrifugal acceleration is obtained from the fifth accelerometer and the sixth accelerometer.
 75. The method as claimed in claim 74 wherein the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.
 76. The method as claimed in claim 75 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the shaft, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the first sensed component, the second sensed component and the third sensed component of the orientation data using the indication of centrifugal acceleration in order to account for a radial misalignment of the first accelerometer, the second accelerometer and the third accelerometer relative to the z-axis.
 77. The method as claimed in claim 76, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a first radial misalignment factor for the first accelerometer wherein the first radial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the indication of centrifugal acceleration, establishing a second radial misalignment factor for the second accelerometer wherein the second radial misalignment factor is expressed as a ratio of a change in the second sensed component to the change in the indication of centrifugal acceleration, and establishing a third radial misalignment factor for the third accelerometer wherein the third radial misalignment factor is expressed as a ratio of a change in the third sensed component to the change in the indication of centrifugal acceleration.
 78. The method as claimed in claim 77 wherein the sensor data processing step is comprised of correcting the first sensed component and the second sensed component of the orientation data in order to account for an axial misalignment of the first accelerometer and the second accelerometer relative to the x-y plane.
 79. The method as claimed in claim 78 wherein the calibrating step is further comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the third sensed component, and establishing a second axial misalignment factor for the second accelerometer wherein the second axial misalignment factor is expressed as a ratio of a change in the second sensed component to a change in the third sensed component.
 80. The method as claimed in claim 60, further comprising the step of processing corrected orientation data comprising at least one set of corrected orientation data to determine the shaft inclination orientation.
 81. The method as claimed in claim 47 wherein the rotation data is comprised of an indication of tangential acceleration of the first member.
 82. The method as claimed in claim 81 wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, wherein the apparatus is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the apparatus is further comprised of a fourth accelerometer for sensing a fourth sensed component in a fourth direction, wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction, and wherein the indication of tangential acceleration is obtained from the first sensed component and the fourth sensed component.
 83. The method as claimed in claim 82 wherein the orientation sensor device is comprised of the first accelerometer, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.
 84. The method as claimed in claim 83 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the first sensed component and the second sensed component of the orientation data using the indication of tangential acceleration in order to account for tangential acceleration of the first member.
 85. The method as claimed in claim 83 wherein the sensor data processing step is comprised of correcting the first sensed component and the second sensed component of the orientation data and correcting the fourth sensed component of the rotation data in order to account for an axial misalignment of the first accelerometer, the second accelerometer and the fourth accelerometer relative to the x-y plane.
 86. The method as claimed in claim 85, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the third sensed component, establishing a second axial misalignment factor for the second accelerometer wherein the second axial misalignment factor is expressed as a ratio of a change in the second sensed component to a change in the third sensed component, and establishing a fourth axial misalignment factor for the fourth accelerometer wherein the fourth axial misalignment factor is expressed as a ratio of a change in the fourth sensed component to a change in the third sensed component.
 87. The method as claimed in claim 83 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the third sensed component of the orientation data using the indication of tangential acceleration in order to account for an axial misalignment of the third accelerometer relative to the z-axis.
 88. The method as claimed in claim 87, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a third axial misalignment factor for the third accelerometer wherein the third axial misalignment factor is expressed as a ratio of a change in the third sensed component to a change in the indication of tangential acceleration.
 89. The method as claimed in claim 47 wherein the rotation data is comprised of an indication of centrifugal acceleration of the first member.
 90. The method as claimed in claim 89 wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, wherein the apparatus is further comprised of a fifth accelerometer for sensing a fifth sensed component in a fifth direction, wherein the apparatus is further comprised of a sixth accelerometer for sensing a sixth sensed component in a sixth direction, wherein the fifth accelerometer and the sixth accelerometer are oriented such that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis and such that the sixth direction is substantially opposite to the fifth direction, and wherein the indication of centrifugal acceleration is obtained from the fifth accelerometer and the sixth accelerometer.
 91. The method as claimed in claim 90 wherein the orientation sensor device is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.
 92. The method as claimed in claim 91 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the first sensed component, the second sensed component and the third sensed component of the orientation data using the indication of centrifugal acceleration in order to account for a radial misalignment of the first accelerometer, the second accelerometer and the third accelerometer relative to the z-axis.
 93. The method as claimed in claim 92, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a first radial misalignment factor for the first accelerometer wherein the first radial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the indication of centrifugal acceleration, establishing a second radial misalignment factor for the second accelerometer wherein the second radial misalignment factor is expressed as a ratio of a change in the second sensed component to the change in the indication of centrifugal acceleration, and establishing a third radial misalignment factor for the third accelerometer wherein the third radial misalignment factor is expressed as a ratio of a change in the third sensed component to the change in the indication of centrifugal acceleration.
 94. The method as claimed in claim 91 wherein the sensor data processing step is comprised of correcting the first sensed component and the second sensed component of the orientation data in order to account for an axial misalignment of the first accelerometer and the second accelerometer relative to the x-y plane.
 95. The method as claimed in claim 94 wherein the calibrating step is further comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the third sensed component, and establishing a second axial misalignment factor for the second accelerometer wherein the second axial misalignment factor is expressed as a ratio of a change in the second sensed component to a change in the third sensed component.
 96. The method as claimed in claim 89 wherein the rotation data is comprised of an indication of tangential acceleration of the first member.
 97. The method as claimed in claim 96 wherein the first member has an axis of rotation, wherein the first member defines an x-y plane perpendicular to the axis of rotation, wherein the first member defines a z-axis coincident with the axis of rotation, wherein the apparatus is further comprised of a fifth accelerometer for sensing a fifth sensed component in a fifth direction, wherein the apparatus is further comprised of a sixth accelerometer for sensing a sixth sensed component in a sixth direction, wherein the fifth accelerometer and the sixth accelerometer are oriented such that the fifth direction and the sixth direction are substantially within the x-y plane and substantially intersect the z-axis and such that the sixth direction is substantially opposite to the fifth direction, and wherein the indication of centrifugal acceleration is obtained from the fifth accelerometer and the sixth accelerometer.
 98. The method as claimed in claim 97 wherein the apparatus is comprised of a first accelerometer for sensing a first sensed component in a first direction, wherein the first accelerometer is offset from the z-axis and is oriented such that the first direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the apparatus is further comprised of a fourth accelerometer for sensing a fourth sensed component in a fourth direction, wherein the fourth accelerometer is offset from the z-axis and is oriented such that the fourth direction is substantially opposite to the first direction, and wherein the indication of tangential acceleration is obtained from the first sensed component and the fourth sensed component.
 99. The method as claimed in claim 98 wherein the orientation sensor device is comprised of the first accelerometer, wherein the orientation sensor device is further comprised of a second accelerometer for sensing a second sensed component in a second direction, wherein the orientation sensor device is further comprised of a third accelerometer for sensing a third sensed component in a third direction, wherein the first direction, the second direction and the third direction are substantially perpendicular to each other.
 100. The method as claimed in claim 99 wherein the second accelerometer is offset from the z-axis and is oriented such that the second direction is substantially within the x-y plane and is substantially tangential to the rotation of the first member, wherein the third accelerometer is offset from the z-axis and is oriented such that the third direction is substantially parallel to the z-axis, and wherein the sensor data processing step is comprised of correcting the first sensed component, the second sensed component and the third sensed component of the orientation data and the fourth sensed component of the rotation data using the indication of centrifugal acceleration in order to account for a radial misalignment of the first accelerometer, the second accelerometer, the third accelerometer and the fourth accelerometer relative to the z-axis.
 101. The method as claimed in claim 100 wherein the sensor data processing step is further comprised of correcting the third sensed component of the orientation data using the indication of tangential acceleration in order to account for an axial misalignment of the third accelerometer relative to the z-axis.
 102. The method as claimed in claim 101 wherein the sensor data processing step is further comprised of correcting the first sensed component of the orientation data and correcting the fourth sensed component of the rotation data in order to account for an axial misalignment of the first accelerometer, the second accelerometer and the fourth accelerometer relative to the x-y plane.
 103. The method as claimed in claim 102 wherein the sensor data processing step is further comprised of correcting the first sensed component and the second sensed component of the orientation data using the indication of tangential acceleration in order to account for tangential acceleration of the first member.
 104. The method as claimed in claim 103 wherein the sensor data processing step is performed iteratively in order to generate the set of corrected orientation data.
 105. The method as claimed in claim 103, further comprising the step of calibrating the apparatus, wherein the calibrating step is comprised of establishing a first radial misalignment factor for the first accelerometer wherein the first radial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the indication of centrifugal acceleration, establishing a second radial misalignment factor for the second accelerometer wherein the second radial misalignment factor is expressed as a ratio of a change in the second sensed component to the change in the indication of centrifugal acceleration, establishing a third radial misalignment factor for the third accelerometer wherein the third radial misalignment factor is expressed as a ratio of a change in the third sensed component to the change in the indication of centrifugal acceleration, and establishing a fourth radial misalignment factor for the fourth accelerometer wherein the fourth radial misalignment factor is expressed as a ratio of a change in the fourth sensed component to the change in the indication of centrifugal acceleration.
 106. The method as claimed in claim 105 wherein the calibrating step is further comprised of establishing a third axial misalignment factor for the third accelerometer wherein the third radial misalignment factor is expressed as a ratio of a change in the third sensed component to a change in the indication of tangential acceleration.
 107. The method as claimed in claim 106 wherein the calibrating step is further comprised of establishing a first axial misalignment factor for the first accelerometer wherein the first axial misalignment factor is expressed as a ratio of a change in the first sensed component to a change in the third sensed component, establishing a second axial misalignment factor for the second accelerometer wherein the second axial misalignment factor is expressed as a ratio of a change in the second sensed component to a change in the third sensed component, and establishing a fourth axial misalignment factor for the fourth accelerometer wherein the fourth axial misalignment factor is expressed as a ratio of a change in the fourth sensed component to a change in the third sensed component. 